Scientists are one step closer to unravelling the mystery of sleep after pinpointing a ‘switch’ in the brain that tells us when to go to sleep and wake up.
The circadian clock reacts to predictable changes like day and night, making sure people sleep at appropriate times, but it doesn’t explain why humans need sleep in the first place.
“That explanation will likely come from understanding the second controller—called the sleep homeostat,” said University of Oxford researcher Professor Gero Miesenböck whose lab conducted the research.”The homeostat measures something – and we don’t know what that something is – that happens in our brains while we are awake, and when that something hits a certain ceiling, we go to sleep. The system is reset during sleep, and the cycle begins anew when we wake up,” he said.
The researchers studied the sleep homeostat in the brains of fruit flies, the same creatures that were used in ground-breaking research into circadian clocks around 45 years ago. “There is a group of cells in the brains of mammals (including, presumably, also people) that have very similar properties to the sleep-promoting cells we have studied in the fly. These cells are also electrically active during sleep, and their activity is stimulated by general anaesthetics,” . These brain cells, or neurons, convey the output of the sleep homeostat – if they are electrically active, the fly is asleep and when they are silent, the fly is awake.
The study, published in the journal Nature, explains how researchers used a technique called optogenetics to switch the brain cell activity on and off with pulses of light. The technique stimulates the production of the neurotransmitter dopamine which is found in increased levels in people who have taken psychostimulants, such as cocaine – an effect that was also seen in the flies.
The researchers discovered an ion channel – which they named ‘Sandman’ – that is kept inside the sleep-control neurons when they are electrically active. Ion channels control the electrical impulses through which brain cells communicate.
When dopamine is present, it causes Sandman to move to the outside of the cell, short-circuiting the neurons and shutting them off – leading the fly to wake up.
Lead author Dr Jeff Donlea compares the device to a home thermostat. “But instead of measuring temperature and turning on the heat when it is too cold, this device turns on sleep when your sleep need exceeds a set point,” he said.
But what is the sleep homeostat measuring? Some theories suggest that it records the depletion of the brain’s energy reserves, or the accumulation of a metabolic waste product. Another suggests that it reacts to information storage capacity in the connections between brain cells.
“We are agnostic about these theories,” Prof Miesenböck told . “We’ll just have to see what we find in our search for the signal that switches the sleep-promoting cells back on.”