It’s that time of the semester again - when four hours per night of sleep rapidly become the norm.  The time when we regularly find at least half-dozen bleary-eyed students clenching their cappuccinos in the library all-night long, trying to crank out last-minute papers.  We know that in humans, extended sleep deprivation leads to mood alteration, decreased energy, and varying degrees of psychosis.  We know that sleep comes in multiple varieties: REM sleep and long-wave (non-REM) sleep.  But why did sleep come about in the first place? Jerome Siegel offers an evolutionary perspective on the function of sleep in October’s Nature Reviews.  He proposes that sleep is a modification of the dormant states evident throughout the animal and plant kingdoms, and that it has adaptive advantages, including optimally timing behavior.

The human brain devours energy, making up merely ~2% of body weight, but consuming ~20% of our energy budget.  Studies have shown that non-REM sleep can reduce the brain’s energy consumption by 30%, which was a considerable evolutionary advantage when our ancestors had to hunt and gather for every calorie.  Moreover, sleep marks a period of forced inactivity, reducing risks of injury and predation.  We can spot a problem with this mental dormancy though, namely that it slows down the brain’s "reboot” time.  REM sleep provides periods of cortical activation and sensory responsiveness that could facilitate alertness upon waking.

Interestingly, we know that animals can rebound from short periods of sleeplessness in ~30% of the time that it would normally take.  This is explicable if sleep serves the purposes of increasing animals’ efficiency of action, although it does beg the question - "why couldn’t we just sleep less?” From an evolutionary perspective, a short sleep rebound would allow mental processes that occur best during sleep to catch up without hampering overall activity levels.  Sleep has been proposed for a variety of neuro-maintenance functions, such as learning and neurogenesis, but it remains unclear whether these can only take place during sleep.  So, while this knowledge may not help next time an all-night term paper rolls around, at least take comfort that sleep may have evolved to increase our efficiency, not to hamper our labors.

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Siegel, Jerome M. "Sleep viewed as a state of adaptive inactivity.” Nature Reviews  Neuroscience 10 (October 2009): 747-753