3. Daily dynamics of states of vigilance in mice

Abstract

Previous studies demonstrated that (a) mice preferentially sleep during light hours and they are preferentially awake during dark hours; (b) mice have fragmented sleep. Despite multiple investigations involving sleep-wake transitions in mice, the exact patterns of sleep-wake alternations are not known. Here we studied sleep-wake pattern distribution in 5 young adult C57BL/6 mice over 5 days using continuous LFP and EMG recordings with a resolution of 5 s. The states were automatically detected based on LFP delta power, theta power and EMG power. We found that daily distribution of states of vigilance in mice was: 12 h of wake, 10.2 h of slow-wave sleep and 1.8 h of REM and it was characterized by several hundred of transitions between different states of vigilance. The majority of detected states lasted less than 40 s. The fragmentation of states of vigilance was higher in the light part of the day and lower in first several hours of the dark part of the day when wake episodes were longer and REM sleep was almost absent. The most stable state of vigilance (longest stable segments) in mice was slow-wave sleep. The main EEG events that contribute to the delta power are slow waves. The existing methods of automatic slow wave detection were not sufficiently robust for their effective automatic recognition in non-anaesthetized mice. We developed a new method for slow wave detection based on neural network pattern recognition and classification, which allowed to retrieve various features of the slow waves in large amounts of data. As expected, we found many slow waves during slow-wave sleep. Surprisingly, isolated slow waves were also present during wake and occasionally during REM sleep. Both delta power and number of slow waves per second were the highest at the beginning of the light cycle, then gradually decreased to the end of the light cycle and reached their minimum in first 3–4 h of the dark cycle. Even though the slow-wave sleep episodes were shorter and occurred less often during dark part of the day, they had higher density in comparison to slow-wave sleep during the light part of the day. We conclude that the results of studies of normal sleep-wake cycle in C57BL/6 mice cannot be directly translated to human research or clinic. Supported by CIHR and NSERC.