Enlarge / This little guy looks too perky to need a nap.Tony Liu
Our sleep is marked by cycles of distinct brain activity. The most well-known of these is probably rapid eye movement, or REM sleep, which is characterized by loss of muscle control leading to twitching and paralysis, along with its eponymous eye movements. REM sleep is widespread in vertebrates, appearing in many mammals and birds; similar periods have also been observed in lizards.
Figuring out what might be going on beyond vertebrates can get a bit challenging, however, as identifying what constitutes sleep isn’t always clear, and many
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