Mixed
Subjective alertness (measured by Stanford Sleepiness Scale) recovers much more slowly than objective psychomotor vigilance, with a time constant of 9.09 hours, making it appear nearly linear over typical sleep durations.
Do not rely on how sleepy you feel to judge your cognitive readiness. Subjective alertness recovers much slower (approx 9 hours to fully asymptote) than your actual ability to react and stay vigilant (approx 2 hours). You may feel subjectively 'fine' after a short nap, but your objective performance may still be recovering. For safety-critical tasks, prioritize objective rest over subjective feeling.
The DRC to the SSS improved much more slowly than psychomotor vigilance, so that it could be fit equally well by a linear function (slope = -0.26) or a saturating exponential function (T=9.09 hours).
Why this rating
Same study design as N1, robust cross-study comparison.
Source
Dose-response Relationship Between Sleep Duration and Human Psychomotor Vigilance and Subjective Alertness
Megan E. Jewett et al. · SLEEP · 1999
DOI 10.1093/sleep/22.2.171
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