Acute Sleep Deprivation on Whole-body Heat Exchange During Exercise-heat Stress in Young and Older Men

Acute Sleep Deprivation on Whole-body Heat Exchange During Exercise-heat Stress in Young and Older Men

The Effect of Sleep Deprivation on Whole-body Heat Loss in Young and Older Men During Exercise in Hot, Dry Conditions

Sleep deprivation has long been thought to modulate thermoregulatory function. Seminal work on sleep deprivation and thermoregulation has demonstrated that sleep-deprived individuals experience greater elevations in core temperature during exercise-heat stress due to reductions in the activation of local heat loss responses of cutaneous vasodilation and sweating. However, it remains unclear 1) if reductions in local heat loss responses would compromise whole-body heat loss (evaporative + dry heat exchange) and 2) if differences exist, are they dependent on the heat load generated by exercise (increases in metabolic rate augments the rate that heat must be dissipated by the body). Further, much of the understanding of the effects of sleep deprivation on thermoregulation has been limited to assessments in young adults. Studies show that aging is associated with reduction in cutaneous vasodilation and sweating that compromise whole-body heat loss exacerbating body heat storage during moderate- and especially more vigorous-intensity exercise in the heat. However, it remains unclear if sleep deprivation may worsen this response in older adults. The purpose of this study is therefore to evaluate the effects of sleep-deprivation on whole-body total heat loss during light, moderate, and vigorous exercise-heat stress and to assess if aging may mediate this response. To achieve this objective, direct calorimetry will be employed to measure whole-body total heat loss in young (18-30 years) and older (50-65 years) men during exercise at increasing, fixed rates of metabolic heat production of 150 (light), 200 (moderate), and 250 W/m2 (vigorous) in dry heat (40°C, ~15% relative humidity) with and without 24 hours of sleep deprivation.

Each participant will complete two exercise trials in a hot, dry environment (40°C, 15% relative humidity) where they will perform three 30 min bouts of exercise at incrementally increasing metabolic heat loads (each separated by a 15-min rest) following a period of either (i) normal sleep or (ii) 24 h of sleep deprivation.

Participants will complete three 30-minute bouts of semi-recumbent cycling at incrementally increasing fixed metabolic heat loads (150, 200 and 250 W/m2) in a hot, dry condition (40°C, 15% relative humidity). Each exercise bout will be separated by a 15 minute period of rest, with the final recovery 1 hour in duration. Exercise will commence between the hours of 7 AM and 9 AM following a period of normal sleep (~8 hours) (Control condition).

Participants will complete three 30-minute bouts of semi-recumbent cycling at incrementally increasing fixed metabolic heat loads (150, 200 and 250 W/m2) in a hot, dry condition (40°C, 15% relative humidity). Each exercise bout will be separated by a 15 minute period of rest, with the final recovery 1 hour in duration. Exercise will commence between the hours of 7 AM and 9 AM following a period of 24 hour of sleep deprivation (Sleep deprivation condition).

Participants will undertake a period of 24 hours of total sleep deprivation prior to completing exercise

Change in body heat storage (i.e., amount of heat stored in the body) calculated as the temporal summation of metabolic heat production and net heat loss

Skin temperature measured continuously at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - bicep, 30%; chest, 30%; thigh, 20%; and calf, 20%.

Change in skin temperature from baseline resting as assessed at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - bicep, 30%; chest, 30%; thigh, 20%; and calf, 20%.

Thermal comfort assessed via a self-report questionnaire upon verbal prompting (4-point scale; 1: comfortable to 4: very uncomfortable)

End of the 30 minute resting baseline period, end of each 30 minute exercise bout, end of each 15 minute rest period, and 1 hour after final exercise period.

Thermal sensation assessed via a self-report questionnaire upon verbal prompting (7-point scale; 0: neutral to 7: extremely hot)

End of the 30 minute resting baseline period, end of each 30 minute exercise bout, end of each 15 minute rest period, and 1 hour after final exercise period.

Thirst sensation assessed via a self-report questionnaire upon verbal prompting (9-point scale; 1: not thirsty at all to 9: very, very thirsty)

End of the 30 minute resting baseline period, end of each 30 minute exercise bout, end of each 15 minute rest period, and 1 hour after final exercise period.

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion)

End of the 30 minute resting baseline period, end of each 30 minute exercise bout, end of each 15 minute rest period, and 1 hour after final exercise period.

Symptoms of tiredness assess via a self-report questionnaire upon verbal prompting (How sleepy are you?) ranging from "feeling active, vital, alert, or wide awake" to "no longer fighting sleep, sleep onset soon, having dream-like thoughts"

End of the 30 minute resting baseline period, end of each 30 minute exercise bout, end of each 15 minute rest period, and 1 hour after final exercise period.

Measures of variability computed from the time, frequency, time-frequency, scale-invariant, entropy, and other nonlinear domains (R-R interval data extracted from the electrocardiogram)

Inclusion Criteria: healthy young (18-30 years) and older males (50-65 years) non-smoking English or French speaking ability to provide informed consent Exclusion Criteria: presence of chronic diseases (e.g., hypertension, diabetes) acute illness (e.g., flu, COVID-19) physical restriction limiting physical activity (e.g., severe arthritis, etc.) use of medication judged by the patient or investigators to make participation in this study inadvisable engaged in regular endurance training

Deidentified participant data will be made available with approved analysis plan and signed access agreement