The brain could have a molecular mechanism capable of tracking the time spent asleep. Researchers observed this in mice. This signal also reflects brief sleep interruptions and the probability of waking up.
The team studied protein kinase A, called PKA. This enzyme adds chemical groups to certain proteins present in brain cells. Its activity had already been associated with the waking state.
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The researchers used a fluorescent sensor to monitor this mechanism in real time. When the mice remained awake, the level of chemical tagging stayed high and stable. It gradually decreased as soon as sleep began.
This decline continued throughout the rest period. It thus formed a kind of molecular timer. The longer the sleep lasted, the weaker the signal related to PKA became.
However, micro-awakenings altered this rhythm. Each very brief interruption was associated with a rise in the signal, before a new decrease. The mechanism therefore recorded both the total duration and the fragmentation of sleep.
According to the researchers, this measurement predicts an imminent awakening better than the duration of rest alone. It does not necessarily trigger awakening. Rather, it indicates that the brain is progressively becoming ready to exit sleep.
This discovery could help better understand sleep debt. This remains difficult to measure directly in a person. Current tests mainly evaluate performance, attention, or physiological consequences of lack of rest.
The study was conducted in mice and published as a preprint. It has therefore not yet been peer-reviewed. Additional work will need to verify whether the same mechanism works in humans.