Two times in the 24-hour sleep cycle are characterized by strong sleep pressure, 4 PM and 4 AM, plus or minus one to two hours. At a certain point sleep pressure is so powerful that an individual will fall asleep regardless of the methods or strategies employed to remain awake.
There are four stages of sleep. These sleep stages move from light to restful to deep. Sleep is broadly characterized by two distinct states, non-REM and REM sleep. These states alternate in 90- to 110-minute cycles. Normal sleep cycles occur 4-5 times throughout the night.
Rapid Eye Movement (REM) sleep, which is characterized by the most vivid dreams, only last 20-25% of total sleep period.
Good sleep can be measured and is called sleep efficiency. Sleep efficiency is a percentage, derived by the amount of time asleep divided by the amount of time spent in bed. A sleep efficiency greater than 90% is an indicator of good sleep.
Sleep latency is the length of time that it takes to transition from completely awake to sleep. Normally, this process takes about ten to fifteen minutes. A latency less than five minutes may be a sign of excessive sleepiness. Normal REM latency ranges from 70 to 120 minutes, with the average young adult REM latency of 90 minutes.