The average adult sleeps 7 1/2 hours a night in five 90 minute sleep cycles. Each of these five sleep cycle is composed of five separate levels of sleep: Alpha, theta, delta, rapid eye movement (REM) and then back to theta. The first three sleep levels last 65 minutes. REM lasts 20 minutes and the final level of sleep lasts 5 minutes. The number of hours you sleep is less important than the number of complete sleep cycles you have when you sleep. Five complete sleep cycles a night is optimal.
Completing less than four sleep cycles a night negatively affects our health. REM sleep is particularly important as it’s primary function appears to be long-term memory storage and restoring oxygen to the cornea. During REM sleep what we’ve learned the day before is transported to the hippocampus. If we do not complete at least four 90 minute sleep cycles a night, long-term memory storage becomes impaired. Completing at least four sleep cycles the night after learning a new skill or the night after studying for a test locks in the new skill or study material. If we get less than four complete 90 minute sleep cycles the night after learning a new skill or the night after studying for a test, it is as if we did not practice the skill or did not study at all because it never fully gets transferred to long-term memory.