The severity of morning brain fog (sleep inertia) is dictated by the depth of the sleep stage from which you are awakened (slow-wave deep sleep), rather than the total duration of your night.

noli's read

The claim that waking up during deep sleep causes more severe sleep inertia is robust and scientifically validated. Experimental laboratory studies, such as the work of Dinges et al. (1985) or Stampi (1990), have shown that waking up during slow-wave deep sleep leads to notable drops in alertness compared to waking up from light sleep. Regarding the '10-minute nap' rule, a randomized controlled trial by Brooks and Lack (2006) confirms that a nap of this short duration immediately improves attention without the risk of drifting into deep sleep. Nevertheless, totally excluding the impact of sleep duration is an exaggeration: observational studies show that sleep deprivation or accumulated sleep debt increases sleep pressure, which worsens inertia upon waking. Furthermore, forced desynchronization protocols prove that circadian timing (your internal clock) has just as much influence on the quality of awakening. Finally, the use of morning light and properly dosed caffeine remains scientifically validated to accelerate the transition to peak alertness.

the sources

No named source held this one up — that absence is part of the verdict.