Sleeping deeply into the afternoon after an all-nighter or a late night out is one way the body repays its sleep debt. The sleep-wake cycle is regulated by a homeostatic process in which the body continuously adjusts its physiological systems to maintain a balanced state of rest and alertness.

A new study identified a specific group of neurons called RE^Vglut2 located in the center of the brain, in the thalamus, that may help us uncover how lost sleep is recovered in animals.

The researchers found that in mice, this circuit, consisting of excitatory neurons, is triggered during sleep deprivation and induces drowsy behavior, followed by deep sleep that can last for hours.

Understanding how neural circuits control sleep homeostasis could provide valuable insights for developing strategies to support shift workers and individuals with disrupted sleep, as well as for addressing diseases like Alzheimer’s that are linked to sleep impairment.

The findings are published in Science.

Sleep is a shared experience across the animal kingdom—a period of reduced physical and mental activity that allows the body to reset and repair, preparing it to carry out all the functions necessary for survival.

The discovery of wake-promoting neurons in the brainstem in the 1940s revealed that certain brain pathways were at the helm of controlling wakefulness. This gave rise to the scientific quest to find the flip side of that coin— what neural circuits or neuron clusters promote sleep-inducing behavior after hours of staying awake?

Sleep has two main phases: rapid eye movement (REM) and non-REM (NREM) sleep. Several studies have identified the neural circuit associated with promoting NREM, which is the more restorative and deeper form of sleep. However, there has been limited success in pinpointing the exact group of neurons responsible for building up sleep needs.

This study identified a specific group of excitatory neurons in the nucleus reuniens (RE) region of the thalamus that was activated during sleep deprivation in mice, using retrograde viral tracing—a neuroscience technique that utilizes a virus as a tracer to map brain connections by tracking its movement from neuron endings back to their cell bodies.

The identified REM neurons send signals to several brain regions known to promote NREM sleep and also influence it by signaling to a specialized region called the zona incerta (ZI).

The researchers carried out optogenetics, where RE neurons in mice were briefly activated using light. They observed that the animals didn’t go to sleep upon activation but exhibited pre-sleep behaviors, such as nesting and grooming, which were followed by a long and deep NREM sleep.

To test whether RE neurons are essential for sleep rebound, researchers chemically inhibited them during sleep deprivation and found that without active RE neurons, the brain’s ability to recover from sleep debt was significantly reduced.

They also found that the RE-ZI connection—critical for the homeostatic regulation of sleep—was weakened, leading to reduced sleep recovery when CaMKII, a key protein that regulates brain plasticity and sleep, was blocked.

By uncovering the role of RE neurons in sleep-promoting behavior after deprivation, the study sets the stage for the discovery of similar sleep-regulating pathways in humans.

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