Diurnal Variation in Subjective and Objective Measures of Sleepiness: the Effects of Sleep Reduction and Circadian Type

In young, good sleepers the diurnal evolution of alertness was studied as a function of degree of morningness: (1) during habitual sleep routine and (2) in a 2-hr sleep reduction protocol. During habitual sleep routine, alertness was assessed using both the subjective evaluation based on Thayer's Activation Deactivation Adjective Checklist (43 subjects) and the objective measurement of sleep latency (Multiple Sleep Latency Test, MSLT). Self-alertness scored highest around midday. Later it showed a dip, then stayed on a plateau until about 2200 hr. On average, 77+ of the subjects fell asleep at the 1400 hr MSLT session while only 35.5+ did at 1000 hr and 25.8+ at 2000 hr. Morning-types (MT) and evening-types (ET) differed only during the morning: ET fell asleep more frequently at 1000 hr and 1200 hr and rated lower self-alertness on arising than did MT. Twelve subjects were given the protocol of a 2-hr sleep reduction (both in delayed bedtime and advanced rising time conditions). At 0700 hr, MT rated their alertness lower when they had only just gotten up (delayed bedtime condition) than when they had been awake for 2 hr (advanced rising time condition). In contrast, ET had the same low level of alertness at 0800 hr, independent of the time elapsed since arising. On average the advanced rising time condition affected the general pattern of alertness more than did delayed bedtime.