Rem sleep, what happens in the brain and body

During rem sleep, your eyes dart beneath closed lids, your brain displays high levels of electrical activity, and the skeletal muscles maintaining posture lose their tone. Rapid eye movement sleep was identified in 1953, when Eugene Aserinsky and Nathaniel Kleitman reported regularly recurring periods of eye motility during sleep, together with the other changes that accompany them. ^[1]

How much rem sleep occurs across a lifespan

In healthy adults, REM takes up roughly a fifth of adult sleep and it is not spread evenly. A meta-analysis of 65 studies covering 3577 people aged 5 to 102 measured the percentage of REM across the lifespan and found it decreases with age in adults, alongside decreases in total sleep time, sleep efficiency and slow wave sleep. ^[2]

However, the decline is not uniform across the two kinds of sleep. In 149 healthy men aged 16 to 83, deep slow wave sleep collapsed between early adulthood and midlife while REM sleep did not change significantly over the same span, and the two therefore follow markedly different chronologies. ^[3]

Why postural muscles become paralyzed

The defining feature besides the eye movements is that postural muscle tone is suppressed, so the body stays still while the brain is highly active. The mechanism is not the obvious one: an experiment in rats showed that blocking the two inhibitory transmitters long assumed to cause it did not abolish the loss of muscle tone, so something else is responsible. ^[4]

CIBDOL · Why postural muscles become paralyzed

What the evidence shows about memory and function

REM has been linked to memory, and the strongest causal evidence is preclinical. In mice, silencing the neurons that generate the theta rhythm during REM, and only during REM, impaired later memory for context while leaving the animals' sleep otherwise undisturbed. ^[5]

The two states appear to do different jobs. A review of the field describes slow wave sleep and REM as supporting different forms of memory consolidation, coordinated by different patterns of oscillation and different levels of neuromodulator activity. ^[6]

What REM is for is still an open question. A review of mammalian sleep sets out the competing accounts, including periodic brain activation, localised recuperation and a role in emotional regulation, without settling between them. ^[7]

Measurement reliability and consumer sensors

REM is also the stage trained humans agree about most. In a reliability programme in which more than 2500 scorers judged 1800 epochs, producing over 3.2 million scoring decisions, overall agreement on sleep stage was 82.6 percent and agreement was highest for REM. ^[8]

A wrist tracker does not measure any of this directly. The figures above come from polysomnography, which records brain activity, eye movement and muscle tone at the same time. Wearable sensors infer sleep architecture indirectly from heart rate variability and accelerometer data. This article reports what REM sleep is and what has been measured about it. It does not offer methods for getting more of it.