Level Of Cognitive Impairment Following Waking Up Depends On Phase Of Biological Clock

Ruzanna Harutyunyan's picture

New research from the Division of Sleep Medicine at Brigham and Women’s Hospital (BWH) and Harvard Medical School finds that sleep inertia, the grogginess and impaired cognitive performance experienced upon awakening from sleep, is much stronger when awakening during the biological night as compared to during the biological day. The findings have implications for people who need to be alert upon awakening including on-call physicians, emergency personnel, pilots and even parents. Conversely, the study shows that waking up from a daytime nap may not be so detrimental to cognitive ability. The research appears in the August issue of the Journal of Biological Rhythms.

The researchers sought to determine whether the magnitude of sleep inertia at the time of awakening varied according to the phase of the internal circadian pacemaker, or "biological clock." Over the course of 11 days spent in the sleep lab, 12 participants were scheduled to live on seven recurring artificial ‘days’ each of 28-hour duration under dim light conditions, while the biological clock ticked along at its inherent rate under these conditions, with a cycle length very close to 24 hours. This technique (called Forced Desynchrony protocol) results in each individual’s sleep/wake cycles being distributed evenly across all phases of the circadian cycle, so that any effect of the circadian cycle on sleep inertia can be assessed.


Each sleep episode included three evenly spaced awakenings to the sound of an alarm clock. Immediately upon awakening, participants completed a series of two-number addition tests (e.g., 51 + 38 = ?). The magnitude of sleep inertia was measured as the increase in the number of correct additions across the first 20 minutes of wakefulness.

Sleep inertia was nearly four times stronger when participants were awoken during the biological night compared to the biological day. Furthermore, subjects showed the least cognitive impairment following awakening during the middle of the biological day. The findings were confirmed when comparing awakening from the same sleep stage.

"Our findings show that sleep inertia has the strongest effect on cognitive function during the biological night, when the core body temperature is at its minimum, and is minimal when awakening during the middle of the biological day. This can have serious implications for people who need to make important decisions upon awakening at different times during day or night, such as emergency personnel, on-call physicians, military personnel and even parents being awoken by a crying baby," said Frank A.J.L. Scheer, a neuroscientist in BWH’s Division of Sleep Medicine and lead author of the paper. He added, "The cognitive impairment during the biological night was twice as large as during the normal time of awakening—the biological morning. This is especially important, considering that already following awakening during the morning the cognitive impairment can be more detrimental than staying awake all night and has been shown to be comparable to the effects of alcohol intoxication."