The Impact of Evening Exercise on Sleep

All our basic physiological functions—such as blood pressure, heart rate, breathing, and digestion—are regulated automatically, without requiring conscious effort. The nervous system responsible for this autonomous control is divided into two major branches that work together to ensure the body responds appropriately to different situations:

– The sympathetic nervous system is activated when energy demands increase, such as in stressful or alert situations (physical exertion or perceived danger), preparing the body to face a physical or psychological challenge. For example, it increases heart rate and the strength of heart contractions, dilates airways to raise blood oxygen levels, triggers the release of stored energy, dilates arteries in the muscles used during activity (including the heart) to boost blood flow, and enhances muscle strength.

– The parasympathetic nervous system, on the other hand, acts as a counterbalance to the sympathetic system, aiming to restore the body’s equilibrium. It slows the heart rate, reduces blood pressure, stimulates digestion to replenish energy reserves, and promotes tissue repair through anti-inflammatory actions. Regular physical activity boosts parasympathetic activity, reflected by a slower resting heart rate and greater heart rate variability.

Parasympathetic Sleep

Sleep is perhaps one of the best examples of the importance of the balance between sympathetic and parasympathetic nervous system activity. Under normal conditions, parasympathetic activity dominates at bedtime, slowing the heart rate and relaxing the muscles; the resulting sleep supports physical and mental regeneration. However, external stimuli such as noise, bright light, potential threats, or mental stress can trigger sympathetic activity, which interferes with these recovery signals and disrupts both the duration and quality of sleep.

This balance between the sympathetic and parasympathetic systems during sleep can also be influenced by physical activity. On one hand, it is well documented that exercise has positive effects on sleep by promoting dominant parasympathetic activity, increasing energy expenditure, reducing stress, and improving mood. On the other hand, it is also clearly established that workouts performed close to bedtime can cause strong sympathetic activation (e.g. increased breathing and heart rate), and the resulting physiological arousal may disrupt sleep. In other words, the timing of physical activity can greatly influence sleep quality—either positively or negatively.

Evening Exercise

For many people, exercising in the evening is the most practical way to fit their favourite sport into their daily routine. According to the results of a recent study, this timing is not incompatible with quality sleep—provided that the activity is completed long enough before going to bed.

In this study, researchers analyzed data collected over one year from 14,689 users of the biometric monitoring platform WHOOP Inc. (Boston, MA), all of whom were at least 18 years old and physically active (engaging in at least 1–2 exercise sessions per week). These wearable devices continuously track several physiological parameters, including heart and breathing rates, heart rate variability, sleep periods, and physical activity levels. Within this cohort, four major categories of physical activity were identified, ranging from light to very intense (Table 1).

Table 1. Levels of Exercise Performed by Study Participants
* Note: Activities lasting less than one minute as well as relaxation exercises such as yoga and stretching were excluded from the analysis. ** HRmax = maximum heart rate. HRmax can be approximately calculated using the formula “220 – age” for men and “226 – age” for women. The numbers in parentheses represent the percentage of total exercise sessions performed by all participants.

The analysis revealed that when exercise ended 4 to 6 hours before the participants’ usual sleep time, sleep onset was similar to that of days without exercise, regardless of the exercise intensity (Figure 1A). However, when exercise ended closer to bedtime, sleep onset became progressively delayed across all levels of exercise intensity, with the effect particularly pronounced for very high-intensity activities.

The delay in falling asleep clearly has a tangible impact on total sleep duration, with a relatively modest loss of about 15 minutes following light activity performed 2 hours before usual bedtime, but this loss can become quite significant, reaching nearly 45 minutes after very intense activity (Figure 2B).

Cardiovascular data analysis indicates that the disruptive effect of exercise performed shortly before bedtime on sleep is due to a reduction in parasympathetic activity. For example, there is a notable increase in resting heart rate, especially among those who performed intense exercise 2–4 hours before their usual sleep onset (Figure 1C), along with a marked decrease in heart rate variability (HRV) (Figure 1D).

A healthy heart doesn’t beat like a metronome—HRV refers to the small, natural variations between heartbeats. These variations increase when parasympathetic activity is dominant, so the observed decrease in HRV after moderate to high-intensity evening exercise suggests that the recovery period was not long enough for the parasympathetic nervous system to fully counterbalance the stimulating effects of exercise on the sympathetic system.

Interestingly, other studies have shown that full parasympathetic recovery after very intense exercise can take more than 24 hours, which could explain the higher resting heart rate and lower HRV even when the intense workout was completed more than 10 hours before bedtime (Figures 1C and 1D).

In summary, these findings indicate that it is possible to mitigate the stimulating effect of exercise on the sympathetic nervous system—and the resulting disruption to sleep—simply by finishing a workout at least 4 hours before one’s usual bedtime.