Feeling exhausted after a long day is an all too familiar part of modern life. We drag ourselves into bed, hoping to shut down our minds for a night, waking up recharged the next day. But contrary to popular belief, your brain does anything but shut down during sleep.
Science is beginning to reveal that sleep is a crucial chance for the brain to consolidate the massive amount of sensory information it receives during the day. It acts as a time-out between periods of consciousness and gives the brain a chance to weave lasting memories from experiences.
For something that is so crucial to our survival, the purpose of sleep is still an enigma to science. It is not simply a question of conserving energy - after all, continually eating doesn't make you feel any more rested. Now, Indrani Ganguly-Fitzgerald and colleagues from the Neurosciences Institute, San Diego, are drawing back the curtain on the function of sleep. And they are doing it by studying that animal so favoured of geneticists, the fruit fly, Drosophila melanogaster (below).
Flies are not known for their smarts, and at first glance, using them to gain insight into the human mind seems like studying a computer by watching an abacus. But at a fundamental level, the minds of flies and humans work in very similar ways, making them ideal for experimentation.
For a start, it is very clear that fruit flies sleep. They show distinct periods of rest, when they are unresponsive to the world around them, and they need more rest if they are deprived of it. They also respond to rich environments similarly to humans by increasing both the size of specific brain regions and the number of connections per nerve cell.
Ganguly-Fitzgerald found that the amount of time that flies spend asleep increases with the amount of experience they acquire while awake. She captured adult flies as they emerged from their pupae and twenty-four of these were thrust, like young debutantes, into rich social environments with at least 30 other peers. Five days later, these social flies were sleeping for about twice as long as loner flies who were kept in solitary confinement. They also preferred more substantial hour-long dozes while the loners were content to catch 15 minute catnaps.
Other experiments confirmed the link between experience and sleep. The more peers the networkers mingled with, the longer they needed to sleep. Light snoozers could be converted into heavy sleepers by giving them more social contact and vice versa And mutant strains that were unable to see or smell received no stimulation no matter what circles they mingled with and always slept for the same amount.
These results could be explained by saying that flies which are more active during waking hours consume more energy, become more tired and need longer to recover. But this was not the whole story. By comparing the brains of different flies, the researchers found that the heavy sleepers had three times as much dopamine - a molecule with roles in memory formation - as the light sleepers.
To test the link between sleep and long-term memory, the team trained amorous flies to be uninterested in the opposite sex, by having their advances repeatedly thwarted by unreceptive females. Sure enough, compared to untrained and still-lustful flies, the trainees slept for 40% longer, but not if they were deprived of sleep after their 'lessons'.
Clearly, snoozing isn't just a chance for the flies to recuperate after a long day. It allowed them precious time to convert their fleeting experiences into long-lasting memories, and it is likely that the same thing happens in humans. We may even discover the origin of dreaming in the activation of mental circuits as the brain plays back the day's activities during the night. Tonight, as you drift off to sleep, you do so not to the jumping of sheep, but to the firing of neurons.