Three frequently asked questions
1) How is sleep recorded in Drosophila?
Sleep monitoring in flies is based on locomotor activity. Usually, this is done using the trikinetics DAM system (http://www.trikinetics.com/): each fly is housed individually in a glass tube (diameter: 5mm, length: 60mm) with food at one end and a foam plug at the other. The number of times the animal crosses an infrared beam in the middle of the glass tube is recorded [1-3]. Infrared video recording methods are also used  (http://www.pysolo.net/). In all cases, periods exceeding 5 minutes without locomotor activity are considered sleep: they are associated with decreased sensitivity to external sensory stimulation, and homeostatic regulation [1, 3]. It is worthy of note that periods exceeding 5 minutes without any locomotor activity have also been evaluated as an 80% reliable marker of sleep in honeybees, although antennae movement are usually used to monitor sleep in this species  (see also question 2). Monitoring locomotor activity to estimate total sleep is not a methodology restricted to insects: in mice maintained in laboratory conditions, a system based on the breaking of infrared beams has also been designed for high throughput sleep monitoring and has been shown to be over 90% accurate . Monitoring locomotor activity is also used for monitoring sleep in the zebrafish .
2) Is there any slow wave sleep and/or REM (Rapid Eye Movement or paradoxical sleep) sleep in flies?
Electrophysiological recording of local field potentials have shown that sleep is associated with changes in neuronal activity in the Drosophila brain [8, 9]. No activity similar to that seen during slow wave sleep or REM sleep has been observed in these studies. One measure of sleep quality in Drosophila is sleep bout duration. During the lights on period (“day”) sleep occurs in short bouts, whereas sleep is more abundant and occurs in longer bouts during the night (lights off). Drosophila is diurnal and most of its sleep occurs during the night. Interestingly, the longest sleep bout of the day is usually observed in the first hour of the night. This is reminiscent of what happens during the first sleep cycle of the night in humans, which is usually characterized by the longest episode of the deepest stage of slow wave sleep. Interestingly, sleep in bees can be evaluated by monitoring the movements of their highly mobile antennae, and several distinct stages in sleep can be defined using this method . So far, no electrophysiological study of cerebral activity during sleep in bees has been reported.
3) How are flies deprived of sleep?
Usually flies are kept in Trikinetics tubes for sleep monitoring (see question 1) and sleep deprivation is carried out by tilting the tubes at regular intervals (several times/ minute) to elicit a locomotor response (negative geotaxis reflex). Automated systems such as the SNAP (Sleep Nullifying Apparatus) have been designed for this purpose . A treadmill like system , or placing two flies in the same tube  have also been used as alternative sleep deprivation methods. Sleep deprivation by any of these methods is followed by a sleep rebound: an increase in sleep over usual daily amounts. This increase is proportional to the amount of sleep lost and reveals the homeostatic regulation of sleep. Constant light  or starvation [13, 14], also reduce sleep but are associated with significant side effects.
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