Where the brain pays sleep debt.

Most animals regularly undergo an astonishing transformation by cycling between wakefulness and sleep. However, if sleep is disrupted or delayed, sleep "debt" accrues, resulting in longer and deeper sleep. Since the discovery of wake-promoting neurons in the brainstem in the 1940s, a hunt for the control mechanisms of sleep-wake behavior has led to a model of reciprocally connected wake-promoting and sleeppromoting cell groups [or nuclei (1, 2)] that achieve discrete brain vigilance states by mutually inhibiting one another (3). But how sleep loss results in rebound sleep to reestablish homeostasis has remained a mystery. On page 1291 of this issue, Lee et al. (4) report a group of neurons in the thalamus (the brain region that relays incoming sensory information to the cerebral cortex) that increases its activity during sleep deprivation and promotes sleep recovery and depth. The findings suggest that these neurons are responsible for promoting sleep homeostasis. [ABSTRACT FROM AUTHOR]