Cirelli, Tononi, and postdoc Giorgio Gilestro report that depriving flies of sleep, either by periodically shaking the vials they call home or by forcing individual male flies to cohabitate with an unwelcome stranger (a male from another fly strain), resulted in higher levels of several synaptic proteins throughout the brain. Levels of these proteins, which included components of the transmitting and receiving sides of the synapse as well as proteins involved in neurotransmitter release, declined after flies had a chance to sleep. This pattern held up even when flies slept at odd hours, confirming that the proteins fluctuate with the sleep-wake cycle, not the time of day...The decrease of synaptic markers during sleep was progressive, and sleep was necessary for their decline. Thus, sleep may be involved in maintaining synaptic homeostasis altered by waking activities.However,
Donlea et al. find that disrupting any one of three genes, including period, an integral component of the circadian clock, prevents flies from sleeping longer after a socially stimulating day. Restoring the genes in just 16 so-called ventral lateral neurons--out of some 200,000 neurons in the fly brain--is enough to restore increased sleep after social enrichment...The circadian clock tells animals when to sleep, but the duration of sleep depends on how long they've been awake and what they've done during that time...the same social experiences that increase the need for sleep also increase the number of synapses between lateral ventral neurons and their partners in the brainstem. After sleep, synapse numbers had declined
...it's unlikely that downscaling happens only during sleep or that synaptic strengthening is limited to waking hours. Human and rodent studies have suggested that sleep may be important for consolidating newly formed memories, a process that's widely assumed to depend on strengthening synapses.