Allow seven to eight hours of sleep, rather than assuming six is enough
Two weeks at six hours a night produced cognitive impairment comparable to two nights without sleep. People did not fully notice: self-rated sleepiness rose in the first few days and then leveled off, with six-hour and four-hour sleepers reporting similar tiredness. Feeling that six hours is enough can itself reflect poor awareness of sleep loss.
What it takes
Free, but reserve another 1–2 hours daily for sleep. Going to bed earlier takes effort.
What you may gain
A randomized sleep-restriction experiment involved 48 healthy adults aged 21–38. Fourteen nights of 6-hour sleep opportunities produced cumulative impairment comparable to 2 nights of total deprivation. Subjective sleepiness rose initially and then barely increased, without a difference between the 6-hour and 4-hour groups. Separately, a meta-analysis of 70 studies of acute sleep deprivation up to 48 hours found effect sizes of g=−0.776 for simple-attention lapses and g=−0.125 for reasoning accuracy.
Context & considerations
Not feeling sleepy does not establish adequate sleep. Habitual six-hour sleepers may adapt subjectively while performance remains impaired. The meta-analysis concerns complete sleep deprivation; the chronic-restriction claim here rests on one randomized trial.
Research & references
Van Dongen, Maislin, Mullington & Dinges (2003). The cumulative cost of additional wakefulness. Sleep. https://doi.org/10.1093/sleep/26.2.117 ; Lim & Dinges (2010). A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin. https://doi.org/10.1037/a0018883