Temperature affects frequency but not rhythmicity of nocturnal awakenings in free-living great tits, Parus major

Several bird species display periodic nocturnal sleep-wake patterns, resembling the ultradian rhythms expressed by mammals. Although relatively little is known about the underlying molecular properties of ultradian biological clocks, field observations demonstrate that the frequency of nocturnal awakenings (which may relate to rhythmicity) increases with ambient environmental temperature. To understand how ambient environmental temperature conditions affect the nocturnal sleep-wake pattern of birds, we experimentally heated nestboxes during the night and monitored the frequency and rhythmicity of awakening behaviour of roosting great tits. More than 80% of great tits displayed ultradian rhythmicity in major nocturnal awakenings, with awakenings occurring every 50-170 min. Experimental increases in temperature, on average, 5 °C, caused birds to wake up approximately 30% more frequently over the course of the night, with the strongest temperature effect occurring during the first part of the night. However, the period length of the predominant nocturnal awakening rhythm was unaffected by increased temperature, likely because most additional awakenings were arrhythmic and clustered during the beginning of the night. We suggest that short-duration awakenings elicited primarily during the first part of the night may not be regulated by an ultradian biological clock, and may respond directly to current environmental conditions, such as the risk environment. Longer-duration awakening bouts, which were not affected by experimental heating, may instead be endogenously regulated by an ultradian clock to optimize clock-controlled sleep patterns and sleep homeostasis.