Previous studies have indicated that manipulation of activity levels can modify characteristics of sleep/wake and activity rhythms. The generality of these observations was evaluated by simultaneously measuring drinking and sleep/wake rhythms while mice had free or no access to a running wheel in constant conditions (DD). Robust circadian rhythms in all parameters were observed in the “wheel free” (unrestrited) condition. When wheels were locked, the peak amplitude of the sleep/wake circadian rhythm decreased by approximately 50% without affecting the amplitude of the drinking rhythm. Total wake time decreased 11% per circadian day when wheels were locked with increases in both NREM and REM sleep. Whereas the amplitude of the drinking waveform was unaffected, wheel restriction caused an equivalent increase in period length (τ) for both rhythms. These results indicate that, unlike the generalized effects of activity on τ, activity restriction influences on rhythm amplitude do not generalize to all behavioral and/or physiological variables. This work also supports the notion that activity influences on sleep/wake rhythm amplitude reflect behavioral “masking” rather than a fundamental change in the direct coupling mechanisms of the biological clock.
