We all need sleep. Miss a night and you really feel the consequences. Sleep contributes to the health and survival of most organisms and sleep disorders contribute to a wide range of physical disturbances. However, studying sleep patterns in mammals is challenging, for subjects and scientists alike. Fortunately, sleeping Drosophila share a lot of similarities with sleeping mammals: they arouse in response to dopamine and have intrinsic circadian rhythms. James Catterson and colleagues from the University of Edinburgh, have looked at the effect of diet on sleep–wake patterns in Drosophila. By examining the effect of various foods on sleep, the important relationship between what we eat and how we sleep can be unravelled.
Setting about testing the sleep-wake patterns of Drosophila, the team used an automated system that measured activity by recording the number of times flies crossed a laser beam. Adult Drosophila held in the lab are usually fed a standard diet made up of agar and a little bit of sucrose. The team wanted to see whether adding yeast to the standard diet would change the fly's sleep patterns, and what effect additional sugar may have.
Male flies fed a yeast-supplemented diet slept less often, for shorter times, and were more active during the day and the night than flies on a conventional lab diet. This may be because increased nocturnal activity is advantageous to male flies that have an increased sex drive at night. However, the female flies slept more often during the day when they ate the diet with added yeast. The team suspect that these sex-based differences may be due to differences in insulin signalling between males and females.
Looking at flies fed a sugar-supplemented diet, the team found no sex-specific differences, but the additional sugar had altered their sleep patterns. Both sexes stayed up longer during the day and night, which resulted in higher activity levels. However, when the flies were asleep, the sucrose diet did not affect how long they slept, suggesting a disconnection of sleep from exercise.
Next, the team investigated the possible mechanism of yeast-induced arousal in male flies by administering a drug that inhibited dopamine synthesis (responsible for arousal from sleep) to males on the standard lab diet and males on the yeast-supplemented diet. Catterson and his colleagues suspected that reducing dopamine would cause the flies to sleep more and be less susceptible to arousal. Looking at the activity levels of the insects on the normal diet, the team found that they were sleepier; however, the effect was diminished when the flies consumed yeast, suggesting that yeast limited the sleep-inducing effect of decreased dopamine levels.
As dietary sugar caused increased activity but not increased sleep, the team investigated whether flies that were less active would require less sleep. This time, they gave the flies a drug that decreased the amount of energy that the insects' muscles could produce and ultimately how much activity they were capable of. As seen with the sucrose diet, the amount of time that the flies slept was independent of their activity level, as the flies showed reduced locomotion but they did not sleep less. The team suspect that fly sleep does not contribute to metabolic balance, and suggests that food intake and locomotor activity are better regulators of energy balance than sleep.
So yeast in the diet aroused male flies during the day and night, while female flies became sleepier during the day. In contrast, sucrose promoted activity in both sexes but did not make them sleep more; meaning that the amount of sleep required by flies appears to be independent of their energy levels and previous activity. Ultimately, this study illustrates the complex nature of physiology, highlighting that diet has far reaching impacts beyond that of gastrointestinal physiology.