The summarized study found that taking a fish oil supplement was associated with better sleep in a cross-sectional study and, more significantly, that fish oil supplementation improved sleep in a randomized controlled trial. However, there are a few reasons to interpret these findings with caution.

First, the effect on sleep appeared to be fairly small, so it’s possible that most of the participants wouldn’t have noticed a difference in their sleep quality. However, it’s hard to say for sure because the researchers used a nonstandard version of the Pittsburgh Sleep Quality Index to measure sleep quality. As a result, there isn’t an established threshold of clinical significance for this scoring system.

Additionally, the results were adjusted for various potential confounders, including age, gender, physical activity, smoking, medication use, and alcohol consumption. Although this can be a useful strategy to strengthen study results, it can be a double-edged sword. If the researchers don’t preregister their planned adjustments, there’s a risk that they will pick and choose which adjustments to include in order to generate a statistically significant result. In the trial preregistration, there was no clear record of any preplanned adjustments.

The results of the summarized study — that supplementation with fish oil improves sleep quality — would be more compelling if a decent number of other clinical trials agreed with its finding. However, although some evidence indicates that omega-3s (typically from fish oil) can improve sleep metrics in infants and children,^[1] effects in adults appear lackluster.

Here are some of the main studies on this topic:

A randomized controlled trial from 2022 is sometimes cited as evidence that omega-3s can improve sleep quality, but the study data undercut this assertion. In the study, 66 middle-aged men and women with poor sleep quality took fish oil for 12 weeks. Fish oil improved sleep efficiency (the percentage of time spent asleep while in bed) compared to a placebo, but this did not translate into any improvements in total sleep time, the time it took to fall asleep, or how sleepy or refreshed participants reported feeling upon waking.^[2]
Another randomized controlled trial in 80 female nurses found that taking omega-3s for 13 weeks reduced insomnia symptoms. However, insomnia symptoms was one of many secondary outcomes and is therefore more likely to have been a chance finding, rather than a genuine effect of omega-3s.
In contrast, a 12-week randomized controlled trial in 355 women with hot flashes — most of whom had poor sleep quality — found that supplementation with fish oil did not clearly reduce insomnia symptoms or improve sleep quality compared to a placebo.^[3] Although sleep quality scores improved slightly more in the fish oil group (by 0.4 points on a scale of 0 to 21), the difference was neither statistically significant nor clinically significant.
A 26-week randomized controlled trial in 86 participants did not find any compelling effects of fish oil supplementation on markers of sleep quality.^[4] Taking a fish oil supplement with mostly DHA caused the participants to fall asleep faster and more efficiently, but they also reported feeling less energetic, so it’s possible that the quality of their sleep was worse. A fish oil supplement with mostly EPA did not lead to any clear benefits for sleep.
Finally, a 60-day randomized controlled trial in 78 deployed soldiers found that supplementation with omega-3s did not improve sleep quality compared to a placebo.^[5]

All in all, these studies suggest that supplementation with omega-3 fatty acids does not provide consistent benefits to sleep quality. It’s plausible that omega-3s might help improve sleep by reducing pain in the context of certain health conditions (e.g., migraine^[6] and osteoarthritis^[7]), but this hypothesis needs to be investigated further.

Fish oil may improve sleep, but it’s just a drop in the pond Original paper