There was a short BBC documentary series about sleep a few years ago, and one of the episodes focused on a man who slept for ten hours a day and still felt drowsy and without energy. After going through a sleep clinic with all kinds of tests, the conclusion was that he should try to sleep only six hours.
The basic issue was that because he spent ten hours a day in bed, trying to catch up on what he thought was a lack of sleeping causing his tiredness, his body never got tired enough to enter deep sleep. Because he missed his deep sleep cycles, his sleep did not recover his energy properly. So this was a vicious cycle.
After a few weeks of forcing himself to wake up after six hours of sleep his problems were basically solved.
So the thing to remember is that duration of sleep tells you nothing about the quality of sleep.
EDIT: While we're here, I've had a lot of sleeping problems in the past, so here some other tips that I remember from this series, and from my sister who studied chronobiology (the science of internal clocks in biological systems, including sleep cycles):
- Note emphasized enough by this article: the deep sleep cycle is necessary for cleaning your brain from neurodegenerating toxins[0]. Getting that stage three deep sleep is essential for your mental health!
- Don't use alcohol to induce sleepiness: yes, it makes you fall asleep faster, but it also prevents you from the deep sleep. You end up not being refreshed afterwards.
- Train your brain to associate your bed with sleeping. If you can't sleep, leave the bed, do something that's not so stimulating that it keeps you awake like reading a book (avoid that mobile phone at all cost) until you feel sleepy enough to go back. This article singles out teenagers here, which is ridiculous: adults are just as guilty of doing this to themselves.
- Because of the previous point, make sure you don't use your laptop/phone in bed, nor have a TV in the bedroom.
- The reason warm baths can help with falling sleep: a fast drop from a high core temperature to a low temperature triggers drowsiness. So what the bath does is heath you up until you are warm. If you then leave that bath and enter a cold bed that needs to be warmed up this triggers a sleep response. Hot milk probably gives a similar trigger, but I'm sure about that one.
- Fasting can help with jet lag. Specifically: don't eat for at least sixteen hours (drinking water is fine), then eat your first meal at breakfast (tangent: "break fast"). This resets the body clock.
[0] https://www.nih.gov/news-events/news-releases/brain-may-flus...