Why humans and other animals sleep is one of the deep mysteries of physiology. One prominent theory in neuroscience is that “memory consolidation” occur during sleep. It means the brain replays memories “offline” to better encode them.

Sleep is important for re-balancing activity in brain networks that have been disturbed during learning while awake. Such “rebalancing” of brain activity involves homeostatic plasticity mechanisms.

This was first postulated at Brandeis University, and has been thoroughly studied by a number of Brandeis labs including the Turrigiano lab.

Now, a study from the Turrigiano lab published in the journal Cell shows that these homeostatic mechanisms are indeed controlled by sleep and wake, but in the opposite way from that theorized previously: homeostatic brain rebalancing occurs exclusively when animals are awake, and is suppressed by sleep.

These findings raise the possibility that different forms of brain plasticity, those involved in memory consolidation and those involved in homeostatic rebalancing must be temporally separated from each other to prevent interference.

For this work, the Turrigiano Lab had to record many terabytes of data necessary to follow the activity of single neurons without interruption for more than 200 hours. Ultimately, these data revealed that the homeostatic regulation of neuronal activity in the cortex is gated by sleep and wake states.

They found, the homeostatic recovery of activity occurred almost exclusively during periods of activity and was inhibited during sleep. Prior predictions have no role for behavioral state, or that sleeping would account for homeostasis.

Finally, the lab established evidence for a causal role for active waking by artificially enhancing natural waking periods during the homeostatic rebound. When animals were kept awake, homeostatic plasticity was further enhanced.

This finding opens doors to a new field of understanding the behavioral, environmental, and circadian influences on homeostatic plasticity mechanisms in the brain. But many questions still remain to be answered.