Sleep inertia – Waking up feeling more tired than when you went to bed? Why is that? Isn’t sleep supposed to leave you feeling rested and recovered, waking up fresh ready to take on the day? In theory, yes, you should be feeling fresh and upbeat when you wake up.
“Immediately after getting up, irrespective of the hour, one is not at one’s best” – Kleitman 1939 in the first observation of ‘sleep inertia’. Sleep inertia is the term used to refer to the temporary time of distorted wakefulness and impaired cognitive function that is often experienced upon waking.
The Third State of Wakefulness?
We often call it ‘waking up tired’, “I woke up so tired this morning!” I say as I reach for my second coffee thinking nothing of it. Sleep as we know it is governed by two factors – A homeostatic urge for sleep and our circadian rhythm that dictates arousal. The longer we are awake throughout the day, our homeostatic longing for sleep increases and cortisol lowers. From that, cortisol leverages the drop to sleep and wakeful arousal.
In a typical static theoretical model, this would look like a spike of cortisol upon waking, and over the course of the day, alert wakefulness and over time spent awake, a later increase for sleep starts to take over. However, what is commonly observed is less sharp alertness and a more slow, disorientated wakening after a period of rest that slowly comes to in a moderate alertness state. Why? ‘Sleep Inertia’ Sleep Inertia lies in the middle of the two.
Sleep Inertia as a Neural Protective Tool
While sleep inertia upon looking at it doesn’t seem to serve a specific function that we know of, some do feel it more than others, and the theory at current from observations is that it provides a protective transition from the sleep realm to the wakefulness realm. We transition from a dream to a wake state.
From an evolutionary standpoint, it would make sense that our ancestors would need to be sharp and alertly attentive to a threat that they had been woken to, however, during our sleep cycles, we can dip into various stages of consciousness and waking directly up from a deep sleep may cause havoc to our neural circuitry. Regional differences in power across the brain, for example, the posterior section seems to sit more dominantly in delta wave sleep In comparison to other areas of the brain, which may suggest this slower transition state is a protective mechanism.
This was especially evident in a recent study by Vallat et al. where it was notable that the functional connectivity of participants that were woken from stage 3 non-rem/deep sleep had high cognitive connectivity upon being woken between the defaulted mode network and various regions across the brain in comparison to those being woken from stage 2 light sleep. Suggesting that this reduction in connections between areas of the brain and the default mode of wakefulness may be what is seen in the sluggish motor and cognitive association skills in the instance of ‘Sleep Inertia’. 
All About the Blood Flow?
Studies conducted on the waking from sleep state of participants has shown that there is less cerebral blood velocity compared to that of a normal wakeful state of being. That seems pretty evident. However, this can usually last for around 30 mins after waking before it begins to balance back to a waking state velocity. 
In the past, we have spoken about the subarachnoid space, which is between the skull and the brain; the neural soup lies in essentially. The volume of this space increases as we sleep to allow for the more functional flow of the cerebral spinal fluid to flush and act upon its glymphatic flushing motion in a steady-state, which may also indicate why this velocity is less when we sleep. 
Am I Taking too Long to Wake up?
Suppose you are feeling like you’re staying in that inertia state a little too long, Ie. It took you until mid-morning to embrace the sunshine; this could be cause for a more in-depth look at overall sleep hygiene. Are you generally tired but wired, for instance, you feel exhausted, but when you do lie down to sleep, you are feeling alert and wired? This could be potentially an imbalance in cortisol and melatonin ratios, where you aren’t having that natural decline of cortisol towards the evening like it would predictably be assumed to occur during this time.
Broken sleep, often waking up and having scattered sleep rhythms during the night, think of shift workers, and new mums often have random odd hours, this can exacerbate the inertia period of time upon waking. In one study done on cumulative sleep loss, four nights of somewhat disturbed or broken sleep, less than the recommended 8 hours, so a dramatic increase in the time it took for people to come to from the inertia like state. 
Take-Home Insight on Sleep Inertia.
Take notice of how long it’s taking you to feel less like a zombie when you wake up; if it’s taking longer than that first half-hour waking, it’s probably worth looking a little deeper into why. There are saliva cortisol tests that can be looked into; often low-grade inflammation states that we become accustomed to, can also take a toll on cortisol levels throughout the day.
How much rest are you getting? Are you in front of a screen and then cramming in a few hours after midnight only to crawl yourself to the bathroom in 4 hours to get ready for work? Poor sleep/sleep deprivation is accumulative remember; you can’t play catch up but you can set your time allotment correctly for adequate sleep moving forward. 
- Hard to wake up? The cerebral correlates of sleep inertia assessed using combined behavioral, EEG and fMRI measures. Vallat R, Meunier D, Nicolas A, Ruby P Neuroimage. 2019 Jan 1; 184():266-278.
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