The introduction reads as follows; Looking in on a sleep study, you’ll see that the brain is actually very active during sleep, cycling through non-REM stages and ending in REM sleep approximately five times during the night. Transitions between stages are clearly visible as shifts in EEG patterns. Graphs illustrating the human sleep cycle typically present an 8-hour time span, as shown below. But this doesn’t tell the whole story of sleep. The amount of time spent sleeping and the content of our sleep changes across the lifespan. Currently, only two-thirds of U.S. adults get the recommended minimum of 7 hours per night, Liu et al., 2016.
At the upper right is an image of a young woman wearing an electrode cap, with more electrodes taped to her forehead and around her eye. Another image shows a LED screen displaying multiple rows of brain waves. The caption reads, this sleep study participant wears electrodes that will measure her brain waves and body movements during sleep.
Below this is an image of a brainwave. The start of the wave shows many similar, high-amplitude, high-frequency waves, followed by a flatter, irregular wave. An arrow is labeled Sleep indicates the point where the waves change. The caption reads, Looking at brain waves allows us to trace a person’s stage of sleep. Here we can see a clear shift from waking to sleeping patterns., FROM DEMENT & VAUGHAN, 1999.
A graph labeled Human Sleep Stages shows sleep patterns between 1 and 8 hours and the corresponding EEG patterns. The caption reads, A typical night's sleep has 4 or 5 multistage sleep cycles, each lasting approximately 90 minutes. Each cycle includes at least 1 non-REM and 1 REM stage. Pattern and duration of stages differ over the course of the night.
There are five sleep stages plotted on the graph. The cycle times and wave appearances for each are;
Awake at 0 hours and 8 hours; The waves are labeled Alpha and Beta waves and are characterized by medium-amplitude waves of varying frequencies. REM sleep; 4 periods approximately every 90 minutes, with each period longer than the previous. The wave pattern is characterized by medium-amplitude and frequency waves, with periods of higher-amplitude and higher-frequency waves labeled Eye movement phase. Stage N1 sleep; Short periods preceding and following each REM stage. The wave pattern is characterized by low amplitude, moderate frequency theta waves. Stage N2 sleep; Short periods preceding and following the first three REM cycles, then remaining until the next REM cycle starts. The wave pattern is characterized by irregular, erratic wave lengths. A sequence of high-frequency waves is labeled Sleep spindle. A sequence of very high-amplitude, low-frequency waves is labeled K-complex. Delta waves, characterize slow wave sleep. Stage N3 sleep; Long periods between the first three REM periods after falling asleep, then vanishing after that. The wave characteristics are erratic, medium amplitude, and varying frequency waves.
The lower graph shows the amount of non-REM sleep, REM sleep, and waking hours experienced according to age. The caption reads, as we age, we need fewer hours of sleep, and the proportion of time spent in REM diminishes. The X-axis is labeled Age in year and shows values from 0 to 10 in increments of 2, and then 20 to 90 in increments of 10. The Y-axis is labeled Hours and shows values from 0 to 24 in increments of 4.
For the non-REM sleep, the graph starts at 8 hours for age 0 slowly decreasing to 7 hours at age 20, and then slowly decreasing to slightly less than 4 hours at age 90.
For the REM Sleep, the graph starts at slightly over 16, or 8 hours of REM, gradually decreasing to 8, or 1 hour of REM at age 20, then maintaining approximately 1 hour of REM sleep across the graph. A callout to the vertical axis of the graph reads, REM is 50 percent of total sleep time at birth. A callout to the end of the graph reads REM decreases to 20 percent of total sleep by age 85.