My dad takes naps at daytime frequently. He asked me recently if sleeping at daytime actually kill you, since he had been reading medical articles that claim that taking naps increases chance of premature death in adults by 1/3. Is this actually true?
From this article:
Scientists from the University of Tokyo are unsure whether it is the napping itself or an underlying condition which makes people more sleepy, that is driving the effect.
After examining more than 200 studies involved 261,000 participants, they found that severe daytime fatigue was associated with a 56 per cent increased risk of developing diabetes.
And taking a regular daytime nap for an hour or more was found to increase the risk of developing the condition by 46 per cent. Author Dr Tomohide Yamada, from the University of Tokyo, Japan, said: "Excessive daytime sleepiness and taking longer naps were associated with increased risk of type 2 diabetes, with a short nap not increasing this risk. "Daytime napping might be a consequence of night-time sleep disturbance such as obstructive sleep apnea.
So, according to current knowledge, the link to adverse health effects may well be mediated via other medical problems such as sleep apnea causing excessive sleepiness during the day. There is no evidence for a causal link between sleeping more during the day and a higher risk of adverse health effects.
To get to such evidence, one could try to correct for a higher incidence of conditions such as sleep apnea in the group of people who sleep more during the day. But the problem with this is that not enough is known about all such risk factors which also influence sleep, so any residual effect that is left may then be due to missing some factors, or having underestimated the effect the known factors have. Also, if you find that sleeping more during the day has a positive effect, that result could be an artifact of having overcompensated for effects such as sleep apnea.
A better approach would be to do an intervention study where people are asked to sleep during the day. But then one would face other problems. Many people who are not used to sleeping during the day may find it difficult to do so. Also, any negative health effects may not arise until many decades of being subjected to sleeping more during the day. The effect you end up measuring after a few years may not be consistent with long term health effects. E.g. what if sleeping during the day helps you to lower the chances of a heart attack if your arteries are clogged, but the same habit over decades increases the chances of getting such clogged arteries?
That's why it's better to stick to what is known about a healthy lifestyle such as getting a lot of exercise, and going to the doctor if there are issues that prevent one from doing that. If you feel so tired and sleepy during the day that you have to sleep, then that is in itself a good reason to visit the doctor.