This study investigated the effects of variations in sleep pressure on cardiac autonomic activity and body temperature. In a counterbalanced design, twelve, healthy, young subjects (6M and 6F) remained recumbent during 30 hours of wakefulness (high sleep pressure) and 6 hours of wakefulness (low sleep pressure). Both periods of wakefulness were immediately followed by a sleep opportunity and the first two hours of sleep were analysed. During extended hours of wakefulness, a reduction in heart rate (HR) was mediated by a decline in cardiac sympathetic activity (SNS; measured via pre-ejection period, PEP) and the maintenance of cardiac parasympathetic activity (PNS; measured via respiratory sinus arrhythmia, RSA). In subsequent high pressure sleep, PNS activity was amplified and SNS activity was negatively associated with slow wave activity (SWA). Sleep deprivation had no impact on foot temperature (Tf), but it did alter the pattern of change in core body temperature (Tc). A downregulation of cardiac autonomic activity during both extended hours of wakefulness and subsequent sleep, may respectively provide "protection" and "recovery" from the temporal extension of cardiac demand.