Cognitive Recovery of Young Males in Thermoneutral Indoor Environments: Effects of Sleep Restrictions

To explore effects of sleep restriction (SR) on next-morning cognitive recovery of young males under thermoneutral environments, three SR conditions, namely the mild (ending at 01:20), moderate (ending at 02:30) and severe sleep restriction (ending at 03:40), were carried out on participants in a thermoneutral environment. During experiments, the subjective sleepiness, perceived workload, and thermal sensation were surveyed. Electrocardiogram (ECG) data were recoded continuously to conduct the heart rate variability (HRV) analysis. In addition, the Deary–Liewald task (including the Simple Response Time task and the Choice Response Time task), Stroop task and Corsi Block task were completed. Results revealed significant increases in sleepiness and perceived workloads during SR. In addition, mean heart rate reduced significantly during moderate (ΔHR = −9.48, p < 0.05) and severe SRs (ΔHR = −9.69, p < 0.01), although it returned to the baseline level in the next morning. The root mean square of successive differences (RMSSD) was elevated during all SRs (Mild SR ΔRMSSD = 27.34, p < 0.05; Moderate SR ΔRMSSD = 33.06, p < 0.01; Severe SR ΔRMSSD = 24.61, p < 0.05) but reduced to baseline the next morning. Furthermore, the sustained attention (SRT) and selective attention performances (CRT) were impaired significantly under moderate (SRT ΔPI = −0.59, p < 0.05; CRT ΔPI = −0.24, p < 0.05) and severe SR (SRT ΔPI = −0.39, p < 0.05; CRT ΔPI = −0.42, p < 0.01). However, the sustained attention performance was restored the next morning even after severe SR, whereas the selective attention performance remained impaired (ΔPI = −0.36, p < 0.01). Significant reductions were observed in the Stroop task performance only after the severe SR (ΔPI = −0.17, p < 0.05), while short-term memory was slightly affected either during or after all SRs (p > 0.05). The overall cognitive performance reduced significantly after the moderate and severe SRs (Moderate SR ΔOPI = −0.30, p < 0.05; Severe SR ΔOPI = −0.40, p < 0.05), even in the next morning. Findings suggest that cognitive impairments caused by mild and moderate SRs could be partially recovered the next morning, while severe SR produced significant impairments in complex cognitive functions, potentially linked to parasympathetic dysregulation and failure of prefrontal compensatory mechanisms. Preliminary findings from this study offer initial implications for cognitive preservation strategies in office environments after night-time overwork.