Sleep inertia is the scientific term of the post-awakening grogginess, impaired cognitive performance and desire to go back to sleep that you may experience yourself on a regular basis. This article will tell you what science knows about this phenomenon and how circadian rhythm appears to be a critical factor in the equation.
Your alarm clock rings, there’s a day awaiting you… but your brain and body just do not feel ready to get up yet.
While it is counter-intuitive from an evolutionary point of view to be slow and confused after an abprubt awakening, it seems like the brain needs some time to transition from sleep to wakefulness.2 Duration can vary from minutes to hours until the body has regained full performance potential. During this period of sleep inertia, impairments are not only experienced subjectively but are also objectively very real: Cognitive performance with regards to solving additions within the first 15-30 after awakening was shown to be worse than after a full 24 hours of sleep deprivation.3
What causes sleep inertia?
Feeling tired directly after waking up is normal and can be observed even under healthy sleeping conditions.2 However, there are some factors that appear to influence severity and duration of sleep inertia.
Prior sleep loss
It has been shown that sleep after a period of sleep deprivation results in stronger and longer sleep inertia than under habitual conditions.2 You may know from the article Circadian Rhythm and Sleep that there’s a neurotransmitter adenosine responsible for sleep pressure, which increases with the time being awake and decreases during sleep. Adenosine may play a role in sleep inertia, too. This theory is supported by multiple studies showing that caffeine, which blocks adenosine receptors in the brain, alleviates sleep inertia both subjectively and objectively.4
Circadian Time of Awakening
The body’s circadian rhythm has an effect on the difficulty of getting up:1,2 Being woken at biological nighttime results in larger sleep inertia, shown by cognitive impairments being 3.6 times greater than after awakening during biological day.5 Even in settings with prior sleep loss, the level of sleep inertia after a nap varied accordingly with the circadian timing of the nap.2
The underlying mechanics appear to have to do with the body’s thermoregulation: A noteworthy study by Kräuchi et al.6 has shown that the proximal skin temperature gradient (DPG, i.e. temperature at hands and feet minus core body temperature) positively correlates with subjective sleepiness. As in the aforementioned studies, biological night has been defined as the point in time of lowest core body temperature (CBT), which is a marker for circadian rhythm and at the same time the point of highest DPG8, this would explain how circadian nighttime worsens sleep inertia.
Early studies on sleep inertia showed correlations between the depth of sleep and the unpleasantness of awakening. This led to the belief that waking up from deep sleep stages, also called slow wave sleep (SWS), results in greater sleep inertia than waking from lighter sleep that is characterized by dreams and rapid eye movements (REM-sleep).2 Products like sleep-cycle alarm clocks base their value proposition on this assumption. However, some recent studies with more differentiated settings found no significant influence of sleep stage on sleep inertia.2,5,6 Therefore, the link between the two might be more complex than traditionally assumed.2
What can you do?
 Trotti, L. M. (2017, October 1). Waking up is the hardest thing I do all day: Sleep inertia and sleep drunkenness. Sleep Medicine Reviews. W.B. Saunders Ltd. https://doi.org/10.1016/j.smrv.2016.08.005
 Kräuchi, K., Cajochen, C., & Wirz-Justice, A. (2004). Waking up properly: Is there a role of thermoregulation in sleep inertia? Journal of Sleep Research, 13(2), 121–127. https://doi.org/10.1111/j.1365-2869.2004.00398.x
 Werken, M. Van De, GimÉnez, M. C., Vries, B. De, Beersma, D. G. M., Van Someren, E. J. W., & Gordijn, M. C. M. (2010). Effects of artificial dawn on sleep inertia, skin temperature, and the awakening cortisol response: Sleep inertia. Journal of Sleep Research, 19(3), 425–435. https://doi.org/10.1111/j.1365-2869.2010.00828.x
 Hasselberg, M. J., McMahon, J., & Parker, K. (2013, January). The validity, reliability, and utility of the iButton® for measurement of body temperature circadian rhythms in sleep/wake research. Sleep Medicine. Sleep Med. https://doi.org/10.1016/j.sleep.2010.12.011
 Gleeson, M. (1998). Temperature regulation during exercise. International Journal of Sports Medicine, 19(SUPPL. 2). https://doi.org/10.1055/s-2007-971967