Caste-dependent sleep of worker honey bees

Sleep is a dynamic phenomenon that changes throughout an organism's lifetime, relating to possible age- or task-associated changes in health, learning ability, vigilance and fitness. Sleep has been identified experimentally in many animals, including honey bees (Apis mellifera). As worker bees age they change castes, typically performing a sequence of different task sets (as 'cell cleaners', 'nurse bees', 'food storers' and 'foragers'). Belonging to a caste could differentially impact the duration, constitution and periodicity of a bee's sleep. We observed individually marked bees within observation hives to determine caste-dependent patterns of sleep behavior. We conducted three studies to investigate the duration and periodicity of sleep when bees were outside comb cells, as well as duration of potential sleep when bees were immobile inside cells. All four worker castes we examined exhibited a sleep state. As bees aged and changed tasks, however, they spent more time and longer uninterrupted periods in a sleep state outside cells, but spent less time and shorter uninterrupted periods immobile inside cells. Although cell cleaners and nurse bees exhibited no sleep: wake rhythmicity, food storers and foragers experienced a 24 h sleep: wake cycle, with more sleep and longer unbroken bouts of sleep during the night than during the day. If immobility within cells is an indicator of sleep, our study reveals that the youngest adult bees sleep the most, with all older castes sleeping the same amount. This in-cell potential sleep may compensate for what would otherwise indicate an exceptional increase of sleep in an aging animal.