Sleep patterns during rearing under different environmental conditions in juvenile rats

Male Wistar rats were chronically implanted at weaning with electrodes for recording EEG amd EMG. From one month of age, they were randomly assigned to either 24 h or 2 h daily 'enriched' (EC), 'standard' (SC) or 'isolated' (IC) rearing conditions. The 24 h-EC group showed the following changes with respect to both the SC and the IC groups: (a) more quiet sleep time; (b) more active sleep time, and (c) shorter active sleep latency. These differences were evident by the third week of environmental conditioning, became statistically significant by 4 weeks, and continued to increase throughout the rest of the enrichment period. None of the sleep parameters showed any significant differences between the SC and the IC groups. The 2 h-EC rats showed an increase in both quiet sleep and active sleep time during the experimental period, whereas the controls showed the expected decline. Morphological investigations at 3 months of age revealed a significant weight increase in the cerebral cortex, and even more so in the hypothalamus as compared with the control group in the 2 h-EC rats.