The reason for sleep is to repair and it’s important for general health and wellbeing.
If you want to get technical about the hormones and where they come from, read this. If not, skip to the next paragraph. The hypothalamus, midbrain and brain stem produce GABA and adenosine to relax us, these are incredibly important for the falling asleep part and to generally feel relaxed; the pineal gland produces melatonin (sleep hormone which helps regulate your body temperature, blood pressure and hormone levels), then the thalamus and cerebral cortex act together to create dreams (4).
While we sleep, the amygdala (in our brain) increases activity, which suggests a lot of processing emotions and “sorting through” stimulus happens. If you don't know, there are different stages of sleep, the length and pattern of these stages is important for us to feel restored when we wake up. There are three or four stages of non-REM (non-rapid eye movement) sleep where our bodies slow and rest, and then there’s REM (rapid eye movement - super inventive with the names, right?) sleep where we have vivid dreams. We cycle through the stages throughout the night, spending only about 2 hours actually dreaming (4). Slow wave sleep (SWS) encompasses non-REM stages 3 and 4, and this is where it is suggested most recovery occurs (5).
How stress disrupts sleep is largely based on the Hypothalamus-Pituitary-Adrenal (HPA) axis and the subtle changes in dominance between the sympathetic nervous system and the parasympathetic nervous system.
The parasympathetic nervous system (PNS) controls homeostasis and the body at rest and is responsible for the body's "rest and digest" function. The sympathetic nervous system (SNS) controls the body's responses to a perceived threat and is responsible for the "fight or flight" response.
Stress impacts the circadian rhythm due to the higher levels of cortisol and epinephrine (adrenaline) that are activated by catecholamines (a group of neurotransmitters) released immediately after exposure to stressors (2). Glucocorticoids (such as cortisol) and epinephrine, released from the adrenal gland, can act as synchronizers of circadian tissue clocks, meaning that changes in their levels or release times can alter the circadian rhythm.
Because of this change in chemical messengers, one can experience insomnia, more frequent waking during the night, bad dreams, and unrefreshing sleep. These two hormones (cortisol and epinephrine) oppose melatonin and other relaxing hormones to control the sleep cycle. As cortisol is high, melatonin is low, and vice versa.
A change in these hormones levels, such as increased cortisol for a longer period of time, impacts the strength of the effect and timing of release of melatonin, impacting the sleep pattern.
When you don't get the right amount of Slow Wave Sleep vs REM
A review of human sleep change studies found that daily life stress and acute stress, such as an exam, had a similar impact to major stressors such as the death of a loved one, with longer sleep latency (how long it takes to sleep), and lower REM density (i.e. lower repair and relax time) (1). This type of stress also changed the pattern of sleep, reducing SWS in the first cycle, and increasing it in the second cycle. This led to a reduced sleep efficiency, more awakenings, decreased REM and SWS (1).
Purely the worry about going to work the next day decreased SWS. Shift workers had increased sleep latency, decreased SWS and increased REM (1). Now, remember that SWS is where a large part of the repairing comes in. If you don't get properly "repaired" during sleep, you wake up feeling like sh*t. Imagine doing that almost every day, for work, at different times.
This means waking feeling unrefreshed and never getting the normal amount of recovery they would get because of the disrupted sleep cycle.
What's interesting is that physical activity delayed REM latency and increased SWS, likely due to the repair needed from muscle use (1).
It is these differences in the actual sleep cycle itself, that lead to increased fatigue and difficulty focusing during the day - potentially causing further stress and perpetuating the cycle.
Now, there are many factors that play into stress. Having a good, hard look at your life and bedtime routine may identify some of those. You can also try to de-stress in the few hours before bed with meditation, some enjoyable reading, or listening to your favourite music. Try to keep the screens off so your melatonin levels are rising normally too (or put them on warm night lights). If you'd like a post about these resources, let me know.
As an health bonus:
Interestingly, there is also recent evidence regarding the microbiome, where microorganisms and circadian genes interact with each other. This suggests yet another pathway that the microbiome impacts, potentially in that physiological and emotional stress can affect the microbiome composition and lead to inflammation or changes in the chemical messages the gut sends out (3). This is particularly relevant because the microbiome are responsible for the production of various neurotransmitters and metabolites such as GABA, dopamine and melatonin - all of which play a role in the circadian rhythm and sleep onset (3, 4).
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