Sleep inertia is the scientific term for the grogginess, cognitive impairment, and desire to go back to sleep that you may regularly experience upon waking. In this article, you’ll learn what science knows about this phenomenon and why the circadian rhythm seems to be a key 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 out of bed.

Although from an evolutionary perspective, it seems counterintuitive to be slow and disoriented after an abrupt awakening, the brain appears to need some time to transition from sleep to wakefulness [2]. This transition can take anywhere from minutes to hours before the body reaches its full performance potential again. During this period of sleep inertia, impairments are not only subjectively felt but are also objectively very real: studies have shown that cognitive performance in solving tasks 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 be observed even under healthy sleep conditions [2]. However, several factors seem to influence the severity and duration of sleep inertia.

Previous sleep loss

It has been shown that sleep following a period of sleep deprivation leads to stronger and longer-lasting sleep inertia than under normal conditions [2]. The neurotransmitter called adenosine is responsible for sleep pressure. It builds up during wakefulness and decreases during sleep. Adenosine might also play a role in sleep inertia. This theory is supported by several 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 impacts the difficulty of waking up [1,2]: Being woken up during the biological night leads to greater sleep inertia, with cognitive impairments being 3.6 times greater than when waking during the biological day [5]. Even in settings with previous sleep loss, the extent of sleep inertia following a nap varied depending on the circadian timing of the nap [2].

The underlying mechanism seems to be related to the body’s thermoregulation: a notable study by Kräuchi et al. [6] showed that the proximal skin temperature gradient (DPG, i.e., temperature of the hands and feet minus core body temperature) is positively correlated with subjective sleepiness. Since the biological night was defined in these studies as the time of the lowest core body temperature (CBT), a marker for the circadian rhythm, and simultaneously the point of the 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 discomfort of waking up. This led to the belief that waking from deep sleep stages, also known as slow-wave sleep (SWS), causes greater sleep inertia than waking from lighter sleep, which is characterized by dreams and rapid eye movement (REM) sleep [2]. Products like sleep cycle alarms are based on this assumption. However, some more recent studies with more nuanced settings have not found a significant effect of sleep stage on sleep inertia [2,5,6]. Therefore, the relationship between the two might be more complex than traditionally assumed [2].

What can you do?

Aside from the obvious—getting enough regular sleep—sleep inertia seems to be worsened by a delayed circadian clock, as this brings your waking time closer to the minimum of your core body temperature [2]. Advancing your circadian clock (with the help of time cues) thus seems to be a promising proactive measure to reduce your daily morning grogginess.

As an acute countermeasure, caffeine has been shown to reduce sleep inertia both subjectively and objectively [3]. Additionally, measures that reduce the proximal skin temperature gradient (DPG), such as increasing core body temperature or cooling hands and feet, may also help combat subjective sleepiness [3,6]. Dawn simulation (e.g. with the use of wake-up lights) has been shown to accelerate the reduction of skin temperature and lessen subjective sleep inertia [7]. Morning exercise also increases core body temperature [9], making it another helpful strategy to feel more alert.