Scientists are getting closer to unravelling the mystery of sleep, having now figured out what happens in the brain at the exact moment when it falls into a slumber. For some time, researchers have sought to locate the body’s “sleep homeostat”, which tracks certain physiological changes in order to decide when it needs to go to sleep. The authors of a new study in Nature now claim that, while they are still no closer to deciphering exactly what this “internal bookkeeper” measures, they have at least figured out how it flips the sleep switch in the brain.
The team conducted their work using fruit fly brains, which have previously been shown to contain a cluster of neurons in a region called the dorsal fan-shaped body (dFB) that appear to control the transition from wakefulness to sleep. Also found in many mammals – including humans – dFB neurons remain silent when we are awake, but become active when they need to send us to sleep.
In previous studies, researchers have artificially played around with the activity of these neurons, discovering that doing so causes the flies to wake up or fall asleep on cue. However, the mechanisms controlling this fluctuating neural activity have so far remained elusive.
To try and shed light on the matter, the study authors used a technique called optogenetics to artificially stimulate the release of dopamine in the dFB of fruit flies. This, they discovered, caused the neurons in this part of the brain to become activated, causing the flies to fall asleep. Conversely, when dopamine was inhibited, dFB neurons fell silent, causing sleeping flies to wake up.
Looking deeper at the processes involved, the team found that the activity of these neurons was controlled by a particular potassium channel, which regulates the flow of potassium ions across cell membranes in order to alter their current. When dFB neurons are silent, this channel – which the researched have dubbed the “Sandman” channel – is located on the inside of these cells, preventing the influx of potassium ions. However, when dopamine is represent, it migrates to the outside of the neuronal membrane, allowing potassium ions to cross this barrier and activate the neuron.
Rather than throwing sand into our eyes in order to send us to sleep, therefore, it would appear that the Sandman actually hits us with a dose of potassium. According to study co-author Gero Miesenböck, this research answers one of the major questions regarding how the sleep homeostat works. However, he explained in a statement that several other key questions remain unresolved, most notably, “what does the sleep homeostat measure? If we knew the answer, we'd be one giant step closer to unraveling the mystery of sleep.”