Participation in regular exercise has been hailed as an effective intervention for promoting sleep, often promoted as a treatment for sleep-related disorders. Research associates higher levels of physical activity with longer and healthier sleep patterns and the adoption of a moderate exercise routine has shown to significantly improve the sleeping habits of previously sedentary insomniacs (Baekeland & Lasky, 1966; Lira et al., 2011).

A central theory proposed for the function of sleep is to replenish energy stores within the brain as a consequence of glycogen depletion during the energetic demands of wakefulness (Scharfa, 2008). This would explain why an exercise induced increase in metabolic rate therefore results in an increased demand for sleep. An alternative theory advocates the fall in body temperature that follows the transient elevation occurring during increased activity is responsible for sleep induction (Atkinson & Davenne, 2007). 30 minutes of moderate-vigorous aerobic activity is enough to increase the body’s temperature by up to 2 °C for duration of 4-5 hours. The body cools to a lower than basal temperature, stimulating a feeling of sleepiness.

Despite evidence supporting the benefits of exercise on improved sleep patterns, issues such as exercise intensity, duration and time of day complicate the relationship and can actually disrupt sleep and encourage insomnia. Exercise training, albeit strength or aerobic, involves the disruption and breakdown of muscle fibres which induces an inflammatory response. Intensive or prolonged exercise can activate a systemic inflammatory response and the production of ‘stress’ hormones cortisol, adrenaline and noradrenaline in an intensity dependent manor. Repair mechanism signalling cascades are activated and the body prepares for the ‘fight or flight’ in the presence of potential danger (Peak et al., 2005). Adaptation has favoured the release of these hormones to increase alertness and vigilance whilst inhibiting feelings of fatigue. This means that exercise of moderate to high intensity in the evening has the potential to delay sleep. When concerned with prolonged endurance exercise, the release of cortisol functions to preserve the body’s carbohydrate stores, ensuring the constant fuel requirements for the brain are met. The body is placed under considerable stress as energy demands remain high and blood sugar levels remain low. Elevated stress hormones switch on catabolic process to provide alternate fuels such as fatty acids and amino acids from fat and protein breakdown. The amplitude of the stress response can be minimised through regularly ingesting carbohydrates throughout and after the exercise session. Alternatively, when aerobic exercise is performed for durations short enough to not critically lower muscle or liver glycogen (e.g. < 45 min).

The magnitude of the stress response to exercise varies considerably from person to person however, with some individuals being affected by the deleterious effects of intense exercise more than others. This means that the impact on sleep can differ substantially from person to person. To conclude however, it is known that the participation in regular physical activity and exercise is important for optimising health and wellbeing in addition to promoting healthy sleep patterns for the majority of the population. However consideration for time of day, intensity and nutrition before and after exercise should be taken to avoid exercise-induced insomnia, particularly in those more sensitive to stress.