Have dabbled in sleep physiology.
Two things that drive sleep-
1. Circadian rhythm, or body clock. This is stuffed for international pilots as they constantly traverse time zones aka constant jetlag. Meant to be awake when timed to be asleep a few time zones back - circadian rhythm dips normally around 3-4am (the "wall" for those that have done night shift) and 3-4pm for those that follow a standard 24h waking day. Italians smartly deal with this with the afternoon siesta.
2. Sleep debt- the longer the time spent awake, the greater the drive to sleep. This is best paid off with a stage of sleep known as slow wave - brain electrical activity greatly slows down.
Concept of the power nap- no longer than 20mins or so. Refreshing, and brain wave activity does not slow down too much, so you can still function afterwards.
If nap for too long, more likely to enter slow wave sleep. Sleep inertia results on waking. Brain waves at 1 Hz take a while to exceed the 14Hzplus of wakefulness; bit like trying to start a cold 1970s engine on a frosty morning. Most will have experienced and witnessed inertia in action - phone call at 2am that you only vaguely recall, or not at all, the kid who can't be soothed after a night terror, feeling groggy after a 1 hour nap on the weekend. Takes up to 15 mins to get back up to speed. We function best if we wake from rapid eye movement sleep, called active sleep is infants, as the electrical activity is similar to wakefulness. First REM period occurs after 90mins sleep, and cycles every 90mins or so.
The mining industry is up to speed with this stuff, as there's money at stake. Much more valuable than safety apparently.