No abstract

The functions of sleep remain elusive. Extensive evidence suggests that sleep performs restorative processes that sustain waking brain performance. An alternative view proposes that sleep simply enforces adaptive inactivity to conserve energy when activity is unproductive. Under this hypothesis, animals may evolve the ability to dispense with sleep when ecological demands favor wakefulness. Here, we show that male pectoral sandpipers (Calidris melanotos), a polygynous Arctic breeding shorebird, are able to maintain high neurobehavioral performance despite greatly reducing their time spent sleeping during a 3-week period of intense male-male competition for access to fertile females. Males that slept the least sired the most offspring. Our results challenge the view that decreased performance is an inescapable outcome of sleep loss.

Among mammalian species, the time spent in the two main "architectural" states of sleep--slow-wave sleep (SWS) and rapid-eye-movement (REM) sleep--varies greatly. Previous comparative studies of sleep architecture found that larger mammals, those with bigger brains, and those with higher absolute basal metabolic rates (BMR) tended to engage in less SWS and REM sleep. Species experiencing a greater risk of predation also exhibited less SWS and REM sleep. In all cases, however, these studies lacked a formal phylogenetic and theoretical framework and used mainly correlational analyses. Using independent contrasts and an updated data set, we extended existing approaches with path analysis to examine the integrated influence of anatomy, physiology, and ecology on sleep architecture. Path model structure was determined by nonmutually exclusive hypotheses for the function of sleep. We found that species with higher relative BMRs engage in less SWS, whereas species with larger relative brain masses engage in more REM sleep. REM sleep was the only sleep variable strongly influenced by predation risk; mammals sleeping in riskier environments engage in less REM sleep. Overall, we found support for some hypotheses for the function of sleep, such as facilitating memory consolidation or learning, but not others, such as energy conservation.

Change in day length is an important cue for reproductive activation in seasonally breeding animals to ensure that the timing of greatest maternal investment (e.g. lactation in mammals) coincides with favourable environmental conditions (e.g. peak productivity). However, artificial light at night has the potential to interfere with the perception of such natural cues. Following a 5-year study on two populations of wild marsupial mammals exposed to different night-time levels of anthropogenic light, we show that light pollution in urban environments masks seasonal changes in ambient light cues, suppressing melatonin levels and delaying births in the tammar wallaby. These results highlight a previously unappreciated relationship linking artificial light at night with induced changes in mammalian reproductive physiology, and the potential for larger-scale impacts at the population level.

SUMMARY Birds provide a unique opportunity to evaluate current theories for the function of sleep. Like mammalian sleep, avian sleep is composed of two states, slow-wave sleep (SWS) and rapid eye-movement (REM) sleep that apparently evolved independently in mammals and birds. Despite this resemblance, however, it has been unclear whether avian SWS shows a compensatory response to sleep loss (i.e., homeostatic regulation), a fundamental aspect of mammalian sleep potentially linked to the function of SWS. Here, we prevented pigeons (Columba livia) from taking their normal naps during the last 8 h of the day. Although time spent in SWS did not change significantly following short-term sleep deprivation, electroencephalogram (EEG) slow-wave activity (SWA; i.e., 0.78-2.34 Hz power density) during SWS increased significantly during the first 3 h of the recovery night when compared with the undisturbed night, and progressively declined thereafter in a manner comparable to that observed in similarly sleep-deprived mammals. SWA was also elevated during REM sleep on the recovery night, a response that might reflect increased SWS pressure and the concomitant Ôspill-overÕ of SWSrelated EEG activity into short episodes of REM sleep. As in rodents, power density during SWS also increased in higher frequencies (9-25 Hz) in response to short-term sleep deprivation. Finally, time spent in REM sleep increased following sleep deprivation. The mammalian-like increase in EEG spectral power density across both low and high frequencies, and the increase in time spent in REM sleep following sleep deprivation suggest that some aspects of avian and mammalian sleep are regulated in a similar manner.k e y w o r d s bird, evolution, pallium, phylogeny, sleep function, slow-wave activity