Sleep inertia is the scientific term for the fatigue after waking, the impairment of cognitive performance, and the desire to go back to sleep that you might regularly experience yourself. In this article, you will learn what science knows about this phenomenon and that the circadian rhythm seems to be a crucial factor in this equation.

Your alarm goes off, the day is waiting for you… but your brain and body just don’t feel ready to get up.

Although it is evolutionarily counterintuitive to be slow and confused after an abrupt awakening, it seems that the brain needs some time to transition from sleep to wakefulness [2]. The duration can vary from minutes to hours until the body has regained its full performance potential. During this period of sleep inertia, impairments are not only experienced subjectively but are also very real objectively: Studies have shown that cognitive performance in terms of task solving is worse within the first 15–30 minutes after waking than after a full 24-hour sleep deprivation [3].

What causes sleep inertia?

Feeling tired immediately after waking is normal and can also be observed under healthy sleep conditions [2]. However, there are some factors that seem to influence the severity and duration of sleep inertia.

Previous sleep loss

It has been shown that sleep after a period of sleep deprivation leads to a stronger and longer sleep inertia than under normal conditions [2]. The neurotransmitter adenosine is responsible for sleep pressure. It increases with the time awake and decreases during sleep. Adenosine may also play a role in sleep inertia. This theory is supported by several studies that show that caffeine, which blocks the adenosine receptors in the brain, alleviates sleep inertia both subjectively and objectively [4].

Circadian time of awakening

The circadian rhythm of the body affects the difficulty of waking up [1,2]: Being awakened during biological nighttime leads to greater sleep inertia, as evidenced by cognitive impairments being 3.6 times higher than after waking during biological daytime [5]. Even in settings with prior sleep loss, the extent of sleep inertia after a nap varied according to the circadian timing of the nap [2].

The underlying mechanics seem to be related to the body's thermoregulation: A remarkable study by Kräuchi et al [6]. has shown that the distal-proximal skin temperature gradient (DPG, i.e., temperature at hands and feet minus core body temperature) correlates positively with subjective sleepiness. Since in the mentioned studies, biological night was defined as the time of the lowest core body temperature (CBT), which is a marker for the circadian rhythm and also the point of highest DPG [8], this would explain how the circadian night worsens sleep inertia.

Sleep stage

Early studies on sleep inertia showed correlations between the depth of sleep and the unpleasantness of waking. This led to the assumption that waking from deep sleep stages, also known as slow-wave sleep (SWS), leads to greater sleep inertia than waking from lighter sleep characterized by dreams and rapid eye movement (REM sleep) [2]. Products like sleep cycle alarms are based on this assumption. However, some recent studies with more differentiated settings found no significant impact of sleep stage on sleep inertia [2,5,6] Therefore, the relationship between the two may be more complex than traditionally assumed [2].

What can you do about it?

Aside from the obvious – getting enough regular sleep – sleep inertia seems to be exacerbated by a delayed circadian clock, as it causes your wake-up time to be closer to the minimum of your core body temperature [2]. Advancing your circadian clock (with the help of timers) is therefore a promising action you can proactively take to reduce your daily morning grogginess.

As an acute countermeasure, caffeine can reduce sleep inertia both subjectively and objectively [3]. Additionally, actions to reduce the proximal skin temperature gradient (DPG), such as increasing core body temperature or cooling hands and feet (e.g., with a cold shower), can also help against subjective sleepiness [3,6]. It has been shown that simulating dawn (such as with the help of wake-up lights) also accelerates the lowering of skin temperature and reduces subjective sleep inertia [7]. Also, morning exercise leads to an increase in core body temperature [9] and can help you feel more awake.