Exercise for sleep - Part 2
In Part 1, we learned that exercise helps with getting better sleep. In this post, we’ll delve into how exercise affects our ability to sleep.
How does exercise affect sleep biology?
Exercise is simply nature’s best sleep aid. Physical activity does not just make us tired. Exercise sets off a coordinated set of biological signals that affect slow wave sleep, circadian timing, autonomic balance, metabolism, immune and inflammation signaling, and emotional regulation. Through multiple overlapping pathways, exercise directly affects how well we sleep.
Myokines: How Your Muscles Talk to Your Brain
When we contract and stress skeletal muscle during exercise, muscle fibers release signaling molecules known as myokines. Myokines function much like hormones, sending messages near and far to regulate metabolic and other biological functions. In this way, skeletal muscle is the largest endocrine organ in our body, and one over which we have direct control!
Over the past 10-20 years, myokines have emerged as a critical link between physical activity, brain function, and sleep. There are likely other myokine mechanisms yet to be discovered, but so far, there are several key myokines that are involved in sleep regulation.
1. Irisin: Exercise → BDNF
Muscles release Irisin (named for the Greek goddess of the rainbow, Iris) during aerobic and resistance exercise, when the FNDC5 protein is cleaved into its active form. Irisin has several known effects, both on general body metabolism and in the brain.
Irisin boosts BDNF (Brain-Derived Neurotrophic Factor), which boosts slow wave sleep
Irisin gets past the blood-brain barrier and increases BDNF in several brain regions. BDNF is crucial for synaptic plasticity and is strongly linked to deep sleep (slow-wave sleep). BDNF promotes slow-wave sleep through direct action on cortical neurons. (ElGrawani 2023 Cell Reports) Slow wave sleep, particularly continuity and stability of deep sleep, improved in a human study of exercise. (Park 2021 Scientific Reports)BDNF also plays a key role in entrainment or adjustment of the circadian clock in the suprachiasmatic nucleus. (Inyushkin 2023 J Cell Res) Strengthening and reinforcing the circadian clock sets the stage for good, consolidated sleep.
Irisin improves metabolic health
Irisin promotes white fat to become “brown” fat — brown fat is highly metabolically active and burns rather than stores calories. Irisin improves insulin sensitivity and stabilizes glucose. Because metabolic dysregulation decreases sleep efficiency and increases nighttime awakenings, irisin’s metabolic benefits indirectly support better sleep. (Here’s a good review of Irisin’s metabolic effects.)
2. IL-6: The Good (from muscle) vs The Bad (from inflammation)
There are two forms of IL-6 that have very different effects.
Muscle-derived IL-6, ie the myokine: increases anti-inflammatory cytokines (like IL-10).
IL-6 from immune cells: increases systemic inflammation and is associated with sleep disruption.
Exercise-derived myokine IL-6 has multiple benefits, such as
Increased cellular glucose uptake and improved metabolic flexibility
Supports circadian alignment via effects on peripheral clocks
Does not produce the pro-inflammatory sleep-fragmenting effects seen in chronic disease states
Overall, exercise shifts inflammatory/immune signaling toward the anti-inflammatory direction, which stabilizes sleep and reduces nighttime awakenings.
3. Myostatin
Myostatin inhibits muscle growth. Exercise, especially resistance training, reduces myostatin levels.
Lower myostatin is associated with:
increased growth hormone secretion
more anabolic repair during sleep
Animal models link lower myostatin to increased N3 (deep), though there are no human data yet on the effects of myostatin on sleep. We DO know that sleep deprivation and poor quality sleep inhibit effects of myostatin.
Basically, if you want to grow muscle, you need to combine resistance exercise with good sleep, so that reduced myostatin can do its thing.
Autonomic Nervous System Effects: Restoring Balance
Sleep depends on a delicate balance between the sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) branches of the autonomic nervous system. Chronic stress, anxiety, and sedentary behavior tilt this balance toward sympathetic dominance, making it difficult to initiate and maintain sleep.
Regular exercise improves autonomic regulation by:
Increasing vagal tone
Lowering resting heart rate
Improving heart rate variability (HRV)
These changes reflect a nervous system with greater parasympathetic tone that can downshift more effectively at night. Over time, exercise trains the body to transition more smoothly from daytime alertness to nighttime recovery.
This helps explain why exercise is particularly effective for people with hyperarousal-driven insomnia, even when sleep duration does not immediately increase.
Circadian Mechanisms: Exercise as a Timing Cue
Light is the dominant circadian signal, but it is not the only one. Exercise acts as a non-photic zeitgeber, or a time cue that helps strengthen the internal biological clock.
Physical activity influences circadian rhythms by:
Modulating clock gene expression in muscle and peripheral tissues
Affecting melatonin timing indirectly through temperature and autonomic pathways
Reinforcing consistent daily rhythms when performed at regular times
Phase shifting effects through BDNF, as mentioned above.
Mood, Anxiety, and the Sleep–Emotion Loop
Sleep and mental health are tightly interwoven. Anxiety and depression disrupt sleep, and poor sleep amplifies emotional dysregulation. Exercise improves sleep in part by acting directly on mood-related neurobiology.
Key mechanisms include:
Increased BDNF and monoamine signaling
Reduced amygdala reactivity to stress
Improved prefrontal regulation of emotional responses
These changes can help reduce nighttime rumination and physiological hyperarousal, which are two major barriers to falling asleep. In clinical studies, exercise interventions often improve sleep before significant changes in mood scores are observed, suggesting a direct effect rather than a purely secondary one.
Putting It All Together
Exercise helps sleep through multiple overlapping biological systems:
Muscle communicates with the brain via myokines
Neurotrophic (BDNF) signaling increases restorative, slow wave sleep and brain plasticity
Autonomic balance facilitates sleep initiation and stability
Metabolic health improves, which also affects sleep depth
Circadian rhythms are strengthened
Mood and anxiety are biologically buffered
Immune-Inflammation pathways tip towards anti-inflammatory, sleep-promoting actions
Takeaway
The key takeaway is not that more exercise is always better, but that regular movement sends multiple powerful signals to affect sleep biology. This is why
Exercise-sleep benefits occur even in people who don’t lose weight or gain muscle
Deep sleep often improves before subjective sleep quality catches up
Sleep quality improves but not necessarily quantity
Aerobic, resistance, and mind-body trainings all help sleep
When viewed through this lens, exercise is truly the best sleep medicine!
This is that time of year when we resolve to do better. If you don't exercise regularly already, maybe consider giving yourself the gift of myokines for your sleep? Check back soon for Part 3, when we will review the types & timing of exercises best for sleep.
