FRIDAY, Aug. 8 (HealthDay News) -- People are groggier and think less clearly when woken up during their night-time sleep than during than an afternoon nap, a new study shows.

The findings, published in the August issue of the Journal of Biological Rhythms, have implications for anyone who needs to be alert upon awakening in the middle of the night, such as on-call physicians, emergency personnel and even parents.

Researchers found that sleep inertia, the period of grogginess and impaired cognitive performance experienced upon awakening, was nearly four times stronger when people were awoken during the middle of their "biological night" (a period of normal night of sleep) compared to their biological day. The feeling was almost twice as strong during the person's biological morning, the wake-up period following a normal night of sleep. People also showed the least thinking impairment after awakening during the middle of the biological day.

"The cognitive impairment during the biological night was twice as large as during the normal time of awakening -- the biological morning," lead author Frank A.J.L. Scheer, a neuroscientist in Brigham and Women's Hospital Division of Sleep Medicine, said in a hospital news release. "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 alcoholic intoxication."

More information

The National Sleep Foundation has more about sleep and lifestyle.