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Figure: Sleep patterns of young and older adults

Humans cycle through two types of sleep which differ in terms of brainwave activity, eye movement activity and muscle activity — rapid eye movement (REM) sleep, which is characterised by rapid movement of the eyes and dreaming, and deeper, non-rapid eye movement (NREM) sleep, in which there is no dreaming. REM and NREM alternate within one sleep cycle, which lasts about 90 minutes in adults. NREM is further subdivided into four separate stages, which increase in depth. Stages 3 and 4 are the deepest stages of NREM sleep. As we age, the amount of stage 3 and 4 NREM sleep gradually reduces, meaning we get fewer hours of the deepest stages of sleep.

Source  Mander B, Winer J, Walker M. Sleep and human aging. Neuron 2017;94:19–36. doi: 10.1016/j.neuron.2017