All vertebrates sleep at some point during the day or night, including humans. As you tuck yourself in at night for your 8 h of bliss, have you ever wondered why we sleep for that amount of time? Why not 12 h every day, or only 2? Mammals vary greatly in the number of hours they spend sleeping each day. For example, giraffes sleep for as little as 3 h per day, while many primates sleep for 8 h. Scientists actually don't know what causes this variation in sleeping hours across species. To solve this problem, Dr Suzana Herculano-Houzel of the Federal University of Rio de Janeiro, Brazil, hypothesized that the time spent sleeping might be related to differences in neuron density in the brain and the brain's surface area across species.
To understand why neuron density and surface area are important, we must first understand one of the functions of sleep. Sleep allows our bodies to clear metabolites – by-products of our body's daily physiological processes – from our brains. The metabolites accumulate during the day, and while some are toxic, others – like adenosine – signal a need for sleep. These metabolites are washed away during sleep by cerebral spinal fluid, the liquid that coats our nervous systems. However, during waking hours the space between cells and neurons in our brains contracts, so the fluid can only clear metabolites at the surface of the brain and not those further below. Similarly, if the neurons in the brain are densely packed beneath the surface, this makes it even harder for the fluid to clear these metabolites.
Across evolutionary history, mammals have developed larger bodies and brains. Previous researchers have shown that larger brains (with more surface area for metabolite rinsing) and less densely packed neurons (allowing metabolites to be rinsed away more readily) are often linked to less time sleeping. However, they couldn't explain cases where large-brained animals like cattle only sleep for a few hours each day, while primates with similar-sized brains sleep for up to 8 h. Herculano-Houzel made an important prediction, suggesting that it is the ratio between neuron density and the surface area of the brain that is rinsed with spinal fluid during waking hours that determines the amount of time spent sleeping each day. To explore this idea, Herculano-Houzel looked up average daily sleep hours for 24 mammalian species from the literature and related it to their average neuron density and brain surface area. She found exactly what she was expecting: as the ratio between neuron density and surface area increased, so too did the amount of time spent sleeping each day.
Her results explain the previous ‘cattle–primate’ sleep discrepancy, as primates have more densely packed neurons, requiring more sleep each day to clear away metabolites that accumulate faster in their brains compared with those of cattle. She found that this ratio – and not surface area or neuron density alone – most accurately predicts sleep time, making it important to consider the relationship between these two factors in future studies on sleep and brain evolution. Her findings suggest a ‘self-reinforcing spiral’ in the evolution of larger brains and bodies: increased neuron density leads to decreased sleep time, thereby allowing animals more time to look for food to support their larger bodily structures.