Amount of REM Sleep

It is hypothesized that one of the functions of sleep is to consolidate and integrate memories. Broadly, sleep can be classified into periods of regular sleep (non-REM sleep) and irregular sleep (REM sleep), with the latter being associated with memory consolidation and having brain activity similar to that which occurs during a waking state. Abnormal trajectories in the development of REM sleep are also correlated with diffciencies in mental development. Therefore, the amount and quality of sleep, particularly REM sleep, might be involved in cognitive differences not just between humans, but more generally between humans and apes.

Humans, at birth, spend about 44-50% of their sleep in a REM-like state. This is similar to the finding that chimpanzees in the first four months of life, on average, spent about 45% of their sleep in REM (for the recording periods). As humans and chimpanzees age, humans have longer periods of REM sleep compared to chimpanzees. Percentage of REM sleep in humans through development.

Age Percentage of REM:

• < 1 year 22.4
• 1-2 years 16.0
• 2+ years 13.1

Both apes and humans have longer REM periods than monkeys. For instance, 5-8 month old M. nemestrina monkeys spend approximately 16% of sleep in REM sleep.

Another feature that was found to vary was the onset and establishment of rhythmic EEG activity during waking. 

Species Appearance Established:

• Rhesus monkey 15 days 2 years
• Chimpanzee 3 months 30-36 months
• Humans 3-5 months 10-12 years

Chimps do not cry during sleep time. During sleep, both humans and chimps produce facial expressions categorized as smiling. There are differences in sleeping patterns in wild chimpanzees and those raised by human caregivers.