A prominent idea emerging from the study of sleep is that this key behavioural state is
regulated in a complex fashion by ecologically and physiologically relevant environmental
factors. This concept implies that sleep, as a behaviour, is plastic and can be regulated
by external agents and changes in internal state. Drosophila melanogaster constitutes
a resourceful model system to study behaviour. In the year 2000, the utility of the
fly to study sleep was realised, and has since extensively contributed to this exciting
field. At the centre of this review, we will discuss studies showing that temperature,
food availability/quality, and interactions with conspecifics can regulate sleep. Indeed
the relationship can be reciprocal and sleep perturbation can also affect feeding and
social interaction. In particular, different environmental temperatures as well as gradual
changes in temperature regulate when, and how much flies sleep. Moreover, the
satiation/starvation status of an individual dictates the balance between sleep and
foraging. Nutritional composition of diet also has a direct impact on sleep amount
and its fragmentation. Likewise, aggression between males, courtship, sexual arousal,
mating, and interactions within large groups of animals has an acute and long-lasting
effect on sleep amount and quality. Importantly, the genes and neuronal circuits that
relay information about the external environment and internal state to sleep centres
are starting to be elucidated in the fly and are the focus of this review. In conclusion,
sleep, as with most behaviours, needs the full commitment of the individual, preventing
participation in other vital activities. A vast array of behaviours that are modulated by
external and internal factors compete with the need to sleep and thus have a significant
role in regulating it.