Behavioural characteristics of sleep in rats under different light/dark conditions.

In the light, rats tend to sleep with curled-up body and closed eyes, while in the dark they tend to sleep more stretched out and often with open eyes. These differences in posture may be caused by the differences in light intensity or by a diurnal rhythm. To study this, rats were provided with EEG and EMG electrodes for sleep classification and two housing conditions were created. One group of rats was maintained on a light/twilight schedule, while another group lived on a twilight/dark schedule. This way the twilight situations, while having the same absolute light level, differ in meaning because of their relative light level. Several behavioural sleep characteristics of the rat were studied, such as body posture, eye closure, position of the rat in the cage and wall contact. The results show that body posture and wall contact are influenced by the absolute light intensity, while eye closure and probably cage position are related to the circadian sleep-wake cycle which itself shows a diurnal rhythm.