Sleep cycles vary in length. The first ones are shorter and they get longer
and longer throughout the night. Most people are short on REM
(Rapid-Eye-Movement or dream sleep) sleep, and already get enough non-REM or
shallow or total sleep. REM sleep removes depression, anxiety, stress, and mental requirements. Non-REM sleep recovers physical energy and removes sleepiness. You can tell which kind of sleep you are missing by the symptoms.
If your sleep environment is noisy, or you have sleep apnea or snore, you might be sleeping lightly and not getting enough REM sleep. This usually makes you want to sleep more and more, and you still don't feel rested. REM sleep is definitely the limiting factor in sleep deprivation. People can go days without sleep, but can't go without dreaming, even if they sleep without dreaming. After about 48 hours, the mind starts dreaming while you are still awake. Hallucinations and waking-sleepwalking are symptoms of REM-sleep deprivation and the brain trying to catch up on its dreaming.
If you miss sleep, you can recover sleep the next night by sleeping more efficiently, that is you don't have to make up sleep on a one-to-one basis. REM sleep is much more rigid. If you miss REM sleep, you get more REM sleep the next night, and it is pretty much proportional to how much you missed. It may take many nights to recover from a lack of dreaming.
-- Harvey Newstrom <mailto://email@example.com> <http://newstaffinc.com> Author, Consultant, Engineer, Legal Hacker, Researcher, Scientist.