By J. E. Hinners, MD MPH
A lot of popular health articles seem to suggest that morning workouts make for better quality sleep at night compared to evening workouts–which *stinks* if you’re not a morning person.
Is it true?
Contrary to popular belief…
- There doesn’t appear to be sufficient evidence that morning workouts make for better sleep
- It may not matter when you work out if you are looking to maximize your quality of sleep
Morning exercise — theoretical and [supposedly] “evidence-based.” Theoretically, avoiding exercise later at night should help you sleep better because exercise increases body temperature and heart rate–both non-ideal sleeping conditions. There have also been a handful of frequently cited sources that seem to indicate that research has confirmed this finding:
- One often-cited study occurred at Appalachian State University, where 3 different groups performed exercise at 3 different times a day (7am, 1pm, and 7pm). This study reports that the 7am exercise group was associated with 75% more time in deep sleep, 85% more time in light sleep, and 20% more time in sleep cycle frequency. However, this study only included 9 subjects.
- A thesis conducted at the same university reported similar findings. However, this study only included 13 subjects.
Why does the number of subjects matter?
- Small sample sizes reduce the power & reliability of the study findings.
- Sample sizes of 50 or less tend to lead to biased, erroneous conclusions.
What a small sample size means and what it does NOT mean
It should be clear that while studies with insufficient sample sizes might not provide reliable data, the possibility that there is any truth in the conclusions cannot necessarily be ruled out. Perhaps if the same studies were conducted with larger sample sizes, the same conclusions might be found.
Evidence suggesting that time of workout doesn’t affect sleep quality:
- The Sleep Medicine journal released an article by Buman et al. in July of 2014. In this cross-sectional study, 1,000 adult subjects of various ages were investigated on exercise timing and sleep quality. Different exercise times included morning, afternoon, and evening (relative to time of going to sleep). Quality of sleep was measured by self-reported total sleep time, sleep latency, quality of sleep, and waking unrefreshed. Evening exercise was not associated with worse sleep.
- A Chinese study by Yu et al. published in June 2013 came to similar conclusions. 5,997 college-age subjects were divided into both exercising and non-exercising groups. Exercising groups were subdivided into those working out in the daytime and those working out in the evening. Measurements included a physical activity rating scale, a perceived exertion rating, and the Pittsburgh sleep quality index. The conclusion was that there was generally no significant impact of working out in the evening on the quality of sleep; findings suggested that only “vigorous” or “very vigorous” exercise in the evening might be harmful to sleep. However this finding of exercise intensity level affecting quality of sleep is debatable, as explained below.
- A June 2014 study by Brand et al. was published in the Sleep Medicine journal which contradicted Yu’s finding about more vigorous exercise being harmful to sleep compared to less vigorous exercise. Brand measured sleep electroencephalographic recordings in 52 regularly exercising young adults who completed moderate to vigorous workouts at night. Those with higher self-perceived exercise exertion at night had better objectively measured sleep quality. Vigorous evening exercisers experienced deeper sleep, shorter sleep onset times, and fewer awakenings after falling asleep compared to less vigorous evening exercisers.
If you are not a morning person (like me) – take heart that you may not HAVE to get up at the crack of dawn to work out in order to get the best night’s sleep!