Anatomy and physiology

Sleep has an underlying structure composed of an alternating cycle of rapid eye movement (REM) and non‐REM (NREM) sleep (Assefa et al. [9]) and is actively regulated by two separate biological mechanisms: sleep–wake homeostasis and the circadian rhythm, which is entrained to the 24‐hour clock (Carley and Farabi [33]). The recommended sleep time for adults aged 18–64 is 7–9 hours, and the recommended time is 7–8 hours in adults over 64 (Hirshkowitz et al. [85]).