Mixed
Restricting sleep to 4-5 hours per night for seven consecutive nights causes cumulative, escalating deficits in subjective sleepiness, mood disturbance, and psychomotor vigilance performance (specifically increased lapse frequency and duration).
If you restrict your sleep to 4-5 hours a night for a week, your ability to stay alert and perform tasks will get worse every day, even if you don't feel significantly more tired. Your reaction times will slow, and you will make more mistakes (lapses). This is a cumulative effect that builds up over the week.
Sleep restriction resulted in statistically robust cumulative effects on waking functions. SSS ratings, subscale scores for fatigue, confusion, tension, and total mood disturbance from the POMS and VAS ratings of mental exhaustion and stress were elevated across days of restricted sleep (p = 0.009 to p = 0.0001). PVT performance parameters, including the frequency and duration of lapses, were also significantly increased by restriction (p = 0.018 to p = 0.0001).
Why this rating
Randomized controlled trial with strict laboratory control, objective performance measures, and statistical significance.
Source
Cumulative Sleepiness, Mood Disturbance, and Psychomotor Vigilance Performance Decrements During a Week of Sleep Restricted to 4–5 Hours per Night
David F. Dinges et al. · SLEEP · 1997
This is one finding among thousands. Every one is graded and traced to its source, so you can see what the evidence actually supports. Browse the research →