Many young animals, whether human, rat or fruit fly, need lots of sleep. The existence of this trait across diverse taxonomic groups implies some fundamental importance, and some scientists are intrigued enough to take a closer look. What mechanisms allow young animals to sleep so much? And what happens if they sleep less?

To address these questions, Amita Seghal and colleagues (Perelman School of Medicine at the University of Pennsylvania, Philadelphia) first measured sleep parameters in fruit flies of different ages. Young flies, which had just emerged from the pupal stage, slept almost 17 hours per day, and only 20% of them woke up in response to a 30-second pulse of light. In comparison, mature flies (8–10 days after emergence) slept about 12 hours per day, and 80% of them woke up in response to the same 30-second light stimulus (Science 344, 269–274; 2014). Young flies were also more resistant to experimentally induced sleep deprivation.

Seghal's team next tried to identify the pathways regulating this age-based difference in sleep among fruit flies. They found that dopaminergic neurons were hypoactive and that levels of dopamine in the brain were >30% lower in young flies compared with mature flies, in keeping with previous indications that dopamine promotes wakefulness in fruit flies. They further identified a specific subset of dopaminergic neurons that is hypoactive in young flies: those that have projections to the dorsal fan-shaped body, a region of the fly brain that is known to promote sleep. Reduced dopaminergic input to this region allows it to maintain greater activity, leading to longer sleep time and more resistance to arousal and sleep deprivation in young flies.

Finally, the research group evaluated the effects of hyperactivation of dopaminergic neurons on young flies. Sleep deprivation in young flies is known to cause developmental abnormalities including impairment of courtship behavior. Seghal's group confirmed that excitation of dopaminergic neurons in young flies caused lasting deficits in courtship behavior: “the flies spend less time courting,” Seghal said in a press release. Sleep-deprived flies had a copulation frequency of <20% compared with almost 70% in control flies. Sleep deprivation specifically impaired neuronal growth in a brain region involved in the olfactory system of flies.

Although it's not known how they may relate to human behavior, the findings “provide the first mechanistic link between sleep in early life and adult behavior,” according to Seghal.