SCHWARZER, Senior District Judge.
Matthew Head appeals the district court’s grant of partial summary judgment in his action against his former employer, Glacier Northwest (“Glacier”), on his claims for disability discrimination under the Americans with Disabilities Act (“ADA”) and Oregon state law on the basis of disability and record of disability. Head also appeals the district court’s exclusion of lay opinion testimony during the trial on his perceived disability and retaliation claims. Finally, Head challenges the district court’s jury instructions requiring him to demonstrate that Glacier discriminated against him “because of’ his perceived disability and retaliated against him “because” of his request for an accommodation. We have jurisdiction pursuant to 28 U.S.C. § 1291. We reverse the grant of summary judgment, affirm the exclusion of lay opinion testimony, vacate the jury [1057]*1057verdict, and remand for further proceedings consistent with this opinion.
I. FACTS AND PROCEDURAL HISTORY
On June 29, 2001, Glacier, Matthew Head’s employer, terminated him after he got a loader he was operating stuck in the mud. The-loader had to be extracted by another piece of equipment. In terminating Head, Glacier cited damage to the loader in violation of an equipment abuse policy issued in 2000. It was undisputed that Head had received this policy.
In early 2001, prior to his termination, Head was diagnosed with depression or bipolar disorder. Head informed Glacier of this diagnosis. At the time of his diagnosis, Head worked graveyard shift as a barge offloader. Head subsequently missed almost two months of work because of his disability. Head requested, and was granted, a Family Medical Leave of Absence for this time period. Although Head returned to work in May 2001, his doctors restricted him from working more than 12 hours per day or 48 hours per week. The doctors also limited Head to working only the day shift.
After his termination, Head filed numerous claims in federal district court. Of relevance to this appeal were Head’s claims under the ADA and Oregon law for disability discrimination based on Head’s disability, record of disability, or perceived disability, and for retaliation for requesting an accommodation. Glacier moved for summary judgment. In opposition to Glacier’s motion, Head did not submit medical or comparative evidence in support of his claims, but did submit numerous affidavits and other evidence. Ultimately, the district court granted partial summary judgment in favor of Glacier on Head’s discrimination claims based on disability and a record of disability. The court reasoned that Head had failed to demonstrate a genuine issue of material fact regarding substantial impairment because he did not present any medical or comparative evidence to support his claims that his disability substantially impaired any major life activities.
The remainder of Head’s claims, for (1) discrimination under the ADA and Oregon law for perceived disability, and (2) retaliation under the ADA and Oregon law for requesting an accommodation, went to trial. The rest of the issues on appeal relate to facts that occurred during the trial.
During the trial, Head’s counsel asked a lay witness the following question about the incident with the loader that preceded Head’s termination: “Was there anything about the position where Mr. Head was stuck or the location of the loader in the material it was stuck in that caused you to believe that he had violated the equipment abuse policy?” Glacier’s counsel objected. The district court sustained the objection based on its earlier decision that' lay opinion testimony regarding whether Head’s getting the loader stuck constituted equipment abuse would not be allowed. The district court believed that such testimony should not be allowed under Federal Rule of Evidence 701. Accordingly, the witness did not answer counsel’s question.
As the trial neared its conclusion, the court and parties discussed the jury instructions. Of primary concern was whether to give a single-motive “because of’ instruction or a mixed-motive “motivating factor” instruction for Head’s state and ADA-based discrimination and retaliation claims. Relying on Costa v. Desert Palace, Inc.,1 a Title VII case, the district court concluded that single-motive “because of’ instructions were appropriate.2 The jury [1058]*1058instruction for disability discrimination therefore required Head to prove that Glacier terminated him “because of’ his perceived disability. The jury instruction for retaliation due to a request for accommodation therefore required Head to prove that Glacier discharged him solely “because” he requested a reasonable accommodation. The jury returned a verdict in favor of Glacier.
Head now appeals the district court’s partial grant of summary judgment, the district court’s exclusion of lay witness testimony regarding whether he violated the equipment abuse policy, and the jury instructions. Head argues that the district court erred by requiring medical and comparative evidence to substantiate the substantial impairment of major life activities at the summary judgment stage. We agree and reverse and remand as to this issue. Head argues that the district court improperly excluded the lay witness testimony. We disagree and affirm as to this issue. Finally, Head argues that the district court erred by giving “because of’ and “because” jury instructions rather than “motivating factor” instructions. We agree, vacate the jury verdict, and remand as to this issue.
II. SUMMARY JUDGMENT
We review the district court’s grant of summary judgment de novo.3 We only review “ ‘evidence available to the [district] court at the time the motion was made.’ ”4 The facts must be viewed in the light most favorable to Head.5
We hold that Ninth Circuit precedent does not require comparative or medical evidence to establish a genuine issue of material fact regarding the impairment of a major life activity at the summary judgment stage. Rather, our precedent supports the principle that a plaintiffs testimony may suffice to establish a genuine issue of material fact. McAlindin v. County of San Diego6 and Fraser v. Goodale are illustrative.
In McAlindin, we discussed medical evidence, but did not explicitly or otherwise require it.7 In fact, we held that a statement in a declaration by McAlindin alone sufficed to raise a genuine issue of material fact regarding the impairment of a major life activity: his ability to engage in sexual relations.8 Additionally, McAlin-din’s declaration played a central role in our holding that sufficient evidence supported the existence of a genuine issue of [1059]*1059material fact as to his sleeping claim.9 Finally, although we discussed doctors’ evaluations of McAlindin’s inability to interact with others, we held that “[h]is alleged ‘fear reaction’ and ‘communicative paralysis’ are sufficiently severe to raise a genuine issue of material fact about his ability to interact with others.” 10 Granted, this holding regarding the inability to interact with others relied more on medical testimony than did our conclusions regarding the other two major life activities, but we did not require medical testimony.
Fraser also did not require medical or comparative evidence at the summary judgment stage. In Fraser, a diabetic argued that her diabetes substantially limited the major life activities of eating, caring for herself, and thinking and communicating.11 After determining that under certain circumstances eating is a major life activity, we held that Fraser’s presentation of evidence about her demanding and highly difficult treatment regimen precluded a grant of summary judgment.12 The evidence on which we relied consisted primarily of Fraser’s testimony regarding her dietary regime. We also considered some medical testimony that even if Fraser perfectly followed her regime she could still have complications; however, we placed no emphasis on this medical testimony.13 Thus, Fraser indicates that medical testimony may be helpful, but it is not required.
Additionally, in rejecting Fraser’s claim that she was substantially limited in caring for herself and thinking and communicating, we faulted the quantity of the evidence presented, not the nature of it.14 We concluded that although Fraser’s statements showed the effects on these life activities, there was insufficient “evidence that she is so unsuccessful in monitoring her blood sugar levels that she is substantially limited in [these major life activities].”15 Accordingly, Fraser supports the principle that a plaintiffs testimony may suffice to establish a genuine issue of material fact. Consequently, it follows that comparative or medical evidence at the summary judgment stage is not required. We hasten to add that our holding in no way impugns our longstanding precedent that concluso-ry declarations are insufficient to raise a question of material fact.16 To survive summary judgment, an affidavit supporting the existence of a disability must not be merely self-serving and must contain sufficient detail to convey the existence of an impairment.17
Because we conclude that plaintiffs need not supply comparative or medical evidence if they provide other adequate evidence, we must now determine whether Head provided adequate evidence in this case. Thus, to determine whether the grant of summary judgment was appropriate, we review Head’s alleged impairment of each major life activity.18 We conclude that Head has alleged sufficient evidence to demonstrate a substantial impairment in the established major life activities of sleeping, interacting with others, and thinking. Moreover, we conclude that reading is a major life activity, and that [1060]*1060Head has alleged sufficient evidence of a substantial impairment regarding reading.19 Accordingly, we reverse the grant of summary judgment as to each claimed major life activity.
A. Sleeping
Sleeping is a major life activity.20 Head’s declaration alleging great difficulty sleeping at night, with some improvement when using sleep medication, suffices to raise a genuine issue of material fact.21 He explained that:
[even] after getting on medication my sleeping improved but I still periodically had serious problems. I would pass out for a while immediately after getting home from work. But I would not get a full night’s sleep, and then after I woke up I had great difficulty going to sleep when it was time to go to bed for the night. This went on for months. Even with the medication I usually was only able to sleep five or six hours a night, compared to the seven to nine hours a night I slept prior to my diagnosis. I was drowsy during the day due to the medications and lack of sleep. Some nights even with the help of medications I could not get to sleep for hours or even at all.
Under McAlindin, this is sufficient evidence to preclude summary judgment.22 Accordingly, Head has alleged sufficient evidence to demonstrate a substantial impairment in the major life activity of sleeping.
B. Interacting with Others
Interacting with others is a major life activity.23 To demonstrate a substantial impairment in the ability to interact with others, Head “must show that his ‘relations with others were characterized on a regular basis by severe problems, for example, consistently high levels of hostility, social withdrawal, or failure to communicate when necessary.’ ”24 At the summary judgment stage, an “alleged ‘fear reaction’ and ‘communicative paralysis’ are sufficient[] to raise a genuine issue of material fact about [Head’s] ability to interact with others.”25
In McAlindin, the testimony of doctors established that McAlindin was always anxious. This led McAlindin “to constrict outside activities and stay away from crowds, shopping centers and any disagreement with his wife.”26 He was around the house at least 20 hours a day and confined his social activities to his family.27 We emphasized that this occurred “all of the time.”28
In this case, Head’s affidavit alleges that he avoids crowds, stores, large family gatherings, and even doctor’s appointments. Furthermore, Head would not leave the house most weekends before [1061]*1061he was fired, and after he was fired he would not leave the house for weeks on end. Head even avoided telephone interaction unless “there were serious consequences” for not responding to phone calls. Head has not alleged that this behavior occurs all the time, but rather that it occurs “many times” or “most” of the time. Thus, Head has stated facts slightly less severe than those found sufficient in McAl-indin.29 Nonetheless, he has still alleged sufficient facts to show a “substantial limitation” and to avoid summary judgment.30 Accordingly, Head has alleged sufficient evidence to demonstrate a substantial impairment in the major life activity of interacting with others.
C. Thinking
Thinking is a major life activity.31 Fraser instructs that the inability to think three times in five months does not constitute a substantial limitation of the ability to think.32 However, Head has alleged a much more persistent problem with his ability to think than that found insufficient in Fraser.33 Head’s affidavit states the following regarding the ability to think:
My bipolar disorder and/or depression greatly affected my short-term memory and ability to concentrate both before and after I was hospitalized. I could not stay focused on something for more than brief periods. I did not have much of a short-term memory at all. I had to be repeatedly reminded of appointments, or tasks I had to do.... If I looked at written material for too long things just got jumbled in my mind and I would have to stop. I could not sit and focus on an entire television show. In the fall of 2001 I quit school because of my inability to focus or concentrate adequately.
Taken in the light most favorable to Head, these statements indicate that his ability to think was almost constantly impaired at some level. Accordingly, Head has alleged sufficient evidence to demonstrate a substantial impairment in the major life activity of thinking.
D. Reading34
At least one court of appeals has held that reading is a major life activity.35 We have not previously addressed whether reading is a major life activity. We now recognize that reading is a major life activity.
“Federal regulations describe major life activities as including functions ‘such as caring for oneself, walking, seeing, hearing, speaking, breathing, learning, and working.’ ”36 We have recognized that the “illustrative list of major life activities requires the activity only to be of ‘comparative importance’ and ‘central to the life process itself,’ and it need not have a public, economic, or daily character.”37 To be a major life activity, the activity [1062]*1062need not be essential to survival, but rather “of central importance to most people’s daily lives.”38
The ability to read is necessary in many instances to perform major life activities such as caring for oneself, learning, and working. As such, it is of central importance to most people’s daily lives. To be sure, a person will not die merely because he or she cannot read, but that is not the standard. The fact that reading is of comparative importance, and that it is central to most people’s daily lives, establishes that reading is a major life activity.39 Consequently, we hold that reading is a major life activity. Accordingly, to survive summary judgment on his reading claim, Head had to allege sufficient facts to establish a substantial impairment of his ability to read.40
In his affidavit, Head alleged that “[r]eading was especially difficult. I basically did not read for more than three to five minutes at a time. If I looked at written material for too long things just got jumbled in my mind and I would have to stop.” Taking these allegations in the light most favorable to Head, it appears that his ability to read was substantially impaired by an inability to read more than a few minutes at a time. Accordingly, Head has alleged sufficient evidence to demonstrate a substantial impairment in the major life activity of reading.
We hold that Ninth Circuit precedent does not require comparative or medical evidence to establish a genuine issue of material fact regarding the impairment of a major life activity at the summary judgment stage. Under the facts of this case, Head has made the minimal showing necessary to survive summary judgment on his claims for substantial impairment of the major life activities of sleeping, interacting with others, thinking, and reading. Accordingly, we reverse the district court and remand for a determination on the merits of Head’s claims based on disability and record of disability.
III. TRIAL ERROR
A. Lay Opinion Testimony
We review evidentiary rulings for abuse of discretion.41 We cannot reverse the district court’s exclusion of lay witness testimony regarding equipment abuse unless we have a definite and firm conviction that the district court committed a clear error of judgment42 and the error was prejudicial.43
A lay witness may give opinions that are: “(a) rationally based on the perception of the witness, [and] (b) helpful to a clear understanding of the witnesses] testimony or the determination of a fact in issue....”44 In this case, Head argues that the district court abused its discretion by not allowing one of his witnesses to answer the following question: “Was there anything about the position where Mr. Head was stuck or the location of the loader in the material it was stuck in that caused you to believe that he had violated the equipment abuse policy?”
Assuming arguendo that the witness had personal knowledge about whether getting the loader stuck was likely to have resulted from equipment abuse, it is unclear how [1063]*1063the witness’s opinion about that would have been helpful to the jury in this case. The jury was fully capable of comparing the incident involving the loader with the standards in the equipment abuse policy and drawing its own conclusion. Accordingly, we conclude that the district judge properly sustained the objection to the question because an answer would not have been helpful to “the determination of a fact in issue.”45
B. Jury Instructions
Head alleges that the district court erred in giving a “because of’ or “because” instruction as to four of his claims: (1) the state discrimination claim; (2) the ADA discrimination claim; (3) the state retaliation claim; and (4) the ADA retaliation claim. To decide these claims, we must determine the causal standard for each claim and whether the jury instructions appropriately reflected that standard. At its core, the causation standard is a legal question; thus, we review it de novo.46 We review the district court’s formulation of jury instructions for abuse of discretion,47 but error in jury instructions does not require reversal if the error was “more probably than not harmless.”48
As to the state retaliation claim, the causal standard for retaliation claims under Oregon Revised Statute § 659A.109 is whether “the unlawful motive was a substantial and impermissible factor in the discharge decision.”49 As to the remaining jury instruction claims, we must determine what standard for causation the ADA requires and how to explain that standard to a jury.50 We hold that the ADA causation standard is a motivating factor standard.51 Furthermore, we hold that the use of the “because of’ instruction was prejudicial in this case. The analysis below first explains how we determined this causation standard, then explains why the jury instruction in this case should have used the “motivating factor” language, and then why the instruction given was prejudicial.
1. ADA’s standard of causation
Causation analysis under the ADA is really a question of whether the ADA’s use of the causal language “because of,”52 “by [1064]*1064reason of,”53 and “because”54 means that discriminatory and retaliatory conduct is proscribed only if it was solely because of, solely by reason of, or solely because an employee was disabled or requested an accommodation. Although the Ninth Circuit has not answered this question, seven of our sister circuits have held that the ADA causation standard does not require a showing of sole cause.55 The Eleventh Circuit in McNely v. Ocala Star-Banner Corp., an ADA case, thoroughly analyzed this question.
The McNely court analyzed the statutory language,56 legislative history,57 and Supreme Court precedent interpreting “because of’ in Title VII cases.58 It explained that “the ‘because of component of the ADA liability standard imposes no more restrictive standard than the ordinary, everyday meaning of the words would be understood to imply. In everyday usage, ‘because of conveys the idea of a factor that made a difference in the outcome.”59
The McNely court noted that the ADA liability provisions do not contain the word “solely” or any similar terms. It explained that, given the absence of that term from the statute, “unless we can discern a very good reason to read the restrictive term ‘solely’ into two statutory provisions where it is not found, this is a simple case.”60 The court declined to import the “solely” restriction from the Rehabilitation Act, explaining that “[a] liability standard that tolerates decisions that would not have been made in the absence of discrimination, but were nonetheless influenced by at least one other factor, does little to ‘eliminate’ discrimination; instead, it indulges it.”61 Thus, the court concluded that importing the term “solely” would undermine the very purpose of the ADA: “ ‘the elimination of discrimination against individuals with disabilities.’ ”62
[1065]*1065Thus, on the basis of the plain language of the ADA, and with the support of seven other circuits, we conclude that “solely” is not the appropriate causal standard under any of the ADA’s liability provisions. The next question is what that proper standard is. We conclude that a motivating factor standard is the appropriate standard for causation in the ADA context for the reasons discussed below.
We agree with our sister circuits that a “motivating factor” standard is most consistent with the plain language of the statute and the purposes of the ADA.63 Moreover, the “motivating factor” standard comports with our existing precedent.64 For example, in Hernandez we characterized the burden as proving that “disability actually played a role in the employer’s decisionmaking process and had a determinative influence on the outcome.”65 Similarly, in Snead we stated the causal requirement as demonstrating to the trier of fact that “a discriminatory reason more likely motivated the employer.”66 Therefore, we hold that the ADA outlaws adverse employment decisions motivated, even in part, by animus based on a plaintiffs disability or request for an accommodation — a motivating factor standard. Our next inquiry is how jury instructions must reflect this causation standard.
2. Jury instructions required under the ADA
Costa explicates the proper approach to formulating jury instructions in an ADA case:
Once at the trial stage, the plaintiff is required to put forward evidence of discrimination “because of’ a protected characteristic. After hearing both parties’ evidence, the district court must decide what legal conclusions the evidence could reasonably support and instruct the jury accordingly.67
The Costa court laid out two alternatives for the trial judge. This approach reflects the fact that although the statute uses “because of’ language, the ADA plaintiff need not show more than that impermissible motives were a “motivating factor” in any adverse action. The approach also reflects the fact that the evidence in a particular case may not suggest more than [1066]*1066one possible reason for the challenged action.
Under the first alternative in Cos-ta, if the judge determines that the only reasonable conclusion the jury could reach is that discriminatory animus is the sole reason for the challenged action or that discrimination played no role in the decision, the jury should be instructed to determine whether the challenged action was taken “because of’ the prohibited reason.68
The second alternative applies in a case in which the evidence could support a finding that discrimination is one of two or more reasons for the challenged decision, at least one of which may be legitimate. In that case the jury should be instructed to determine whether the discriminatory reason was “a motivating factor” in the challenged action.69
At issue in this case regarding Head’s discrimination claims was why Glacier fired him — whether it did so because he violated the equipment policy, because Glacier perceived him as being disabled, or because of some combination of the two. At issue regarding Head’s retaliation claims was whether Glacier fired him because he violated the equipment policy, because he requested a reasonable accommodation, or because of some combination of the two. There was evidence to support a conclusion that each of these reasons had a role in Head’s discharge.
Thus, a jury could have found that Head was fired for violating the equipment abuse policy, because of his perceived disability, or for both reasons. It was error therefore for the court to refuse to give the requested mixed-motive instruction.
3. Prejudice
The district court’s use of “because of’ and “because” jury instructions in this case “does not require reversal if the error was ‘more probably than not harmless.’ ”70 The harmless error standard requires error to be proven harmless more probably than not,71 which in practical effect means that an error is prejudicial unless evidence more probably than not proves otherwise.
As noted, a jury could have found that Head was fired for violating the equipment abuse policy, because of his perceived disability, or both. Under the correct causation standard, the second or third of these findings should have meant liability for Glacier. But the “because of’ instruction given could have allowed a jury to conclude that Head needed to show sole causation and thus to deny liability for Glacier even though the jury had found that both reasons played a role in motivating Glacier’s discharge of Head. Thus, the instructions given improperly favored Glacier.
The record contains no indication that the jury’s verdict was more likely based on the finding that Glacier’s action was based solely on Head’s performance. Thus, it is not possible to conclude that the error in instructions was more likely than not harmless to Head. Because the instructional error cannot be shown to be harmless, we vacate the jury verdict.
IV. CONCLUSION
We conclude that Head has alleged sufficient evidence to demonstrate a substantial impairment in the established major life activities of sleeping, interacting with others, and thinking. Moreover, Head has [1067]*1067alleged sufficient evidence of a substantial impairment regarding reading and we conclude that reading is a major life activity. Therefore, we reverse the district court and remand for a determination on the merits of Head’s claims based on disability and record of disability. The district court properly excluded lay witness testimony because it would not have been helpful to the jury. Finally, we adopt a “motivating factor” standard for causation in the ADA context and conclude that the use of “because of’ and “because” instructions in this case was prejudicial. Therefore, we vacate the jury verdict and remand for further proceedings.
REVERSED and REMANDED in part, AFFIRMED in part. Each party shall bear its own costs on appeal.