Diurnal animals with monophasic sleep patterns exhibit species-specific cycles of slow-wave sleep (SWS) alternating with rapid eye movement sleep (REMS). A recent study revealed that in the diurnal reptile Pogona vitticeps, SWS and REMS regularly alternate with approximately 2-min cycles throughout the night, generating more than 300 SWS/REMS cycles. Although the cycle varies significantly across species, common neural mechanisms may be involved. However, the precise mechanisms responsible for maintaining these regular cycles remain elusive. Sleep-wake patterns are predominantly regulated by the circadian clock, which maintains an approximately 24-h sleep/wake rhythm even in the absence of external cues. Several studies report that each vigilance stage, notably wakefulness and REMS, relies on circadian rhythms. Nevertheless, to date, few studies have explored circadian effects on the dynamics of REMS/SWS alternation. To address this question, we recorded locomotor activity and neuronal activity of P. vitticeps under constant darkness (DD) conditions. Under these conditions, sleep behavior and sleep-related neuronal activity emerged over a 24-h cycle. However, the regularity of the REMS/SWS alternation was disrupted under these conditions. Notably, when the lizards were then exposed to 12 h of light after DD, the regularity of the sleep stages was restored. These results suggest that sleep-related neuronal activity in lizards is regulated by circadian rhythms and that the regularity of REMS and SWS cycling is influenced by daytime light exposure.