Sleep is divided into 90-minute cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep; these 90-minute cycles are repeated three to six times during the night (ultradian pattern). Sleep deprivation, medications and psychoactive substances (e.g., coffee, nicotine, alcohol), and medical conditions all may modify the timing of usual sleep described in this section.
The time between the onset of sleep and the end of the first REM period defines the first sleep cycle. After that, sleep cycles (i.e., second through sixth cycle) always start with NREM sleep and end in REM sleep. (See Figure 1.2.)
The majority of slow-wave sleep (SWS) occurs in the first third of the night, while the majority of REM (and most dreams) occurs in the last third of the night.
Figure 1.2: Typical sleep pattern of a young human adult.27
The National Sleep Foundation recommends that adults aged 26-64 sleep 7 to 9 hours per night, and adults aged 65 and older sleep 7-8 hours per night28. Healthy sleepers can experience up to 10 short arousals (i.e., awakenings on the order of just seconds long) per sleep hour. These arousals are often associated with some body movements and are typically forgotten — so one is totally unaware of any awakenings in the morning — unless they last for a few minutes, or something unusual occurs during the arousals (or are associated with them), like a specific sound or odor.