Ultradian rhythms in young and adult mice: further support for the basic rest-activity cycle

Basic Rest-Activity Cycle (BRAC) rhythmicity has been shown in many parameters in animals as well as in humans. A similarity between the periods of BRAC and REM/NREM cycles has been frequently reported. However, this does not necessarily prove the idea that BRAC is a diurnal extension of the REM/NREM cycle. A concurrent variation of the BRAC and REM/NREM cycles during ontogenetic development would confirm the link between these rhythms. For this purpose, we investigated the existence of a BRAC periodicity in young (23 days of life) and adult (90 days of life) mice, in comparison with the REM/NREM species-specific cycle. Reaction times of the avoidance response to a visual warning stimulus that preceded an electric shock were measured by a computer-controlled shuttle-box. Results revealed the existence of an evident ultradian rhythm in all subjects confirming previous reports of a 20-min BRAC in avoidance behavior of adult mice. Furthermore, young mice showed a shorter periodicity of about 9 min, which is analogous to their 10-min REM/NREM cycle. These results seem to confirm the hypothesis of a strong relationship between BRAC and REM/NREM sleep cycle.