Meet the Jetsons
I just spent the evening hosting my former Ph.D. student Andreas, who’s returning from Greece to Canada for his convocation. His family and him were a bit sleepy and jet lagged, to say the least. In between making shots of extra-strong espresso for everybody, it got me to thinking about just what the effects of sleep deprivation are on the human body and its exercise capacity.
Sleep deprivation is possibly one of the most insidious health issues facing modern society as a whole. With advances in technology enabling increased work demands in and out of the office along with family and social responsibilities, sleep often becomes the first casualty in time management for busy professionals and recreational athletes. Sleep deprivation becomes extremely problematic in occupational settings due to the commonality of shift work in many professions.
Furthermore, the risk of accidents occurring due to sleep deprivation-related fatigue can pose severe hazards beyond the worker. Chernobyl and the Exxon Valdez are all major disasters where sleep-deprived fatigue contributed to inappropriate, and ultimately catastrophic, responses. Driving while drowsy also leads to many tragic traffic accidents, including the recent death of two cyclists in California from a sherriff’s patrol car.
Long Haul Athletes
Whether it’s new ProTour events requiring teams to send riders to Australia in January, or Astana getting a last-minute invite to the Giro, cyclists are now expected to travel across the globe for training and competition. Astana had to mobilize its team on a week’s notice to get to Sicily, including flying Levi Leipheimer from California across 8+ time zones.
The fatigue and jetlag associated with such prolonged travel can lead to a multitude of problems for athletes. The primary problem, of course, is problems with getting adequate quantity of quality sleep. Additional problems range from simple discomfort and fatigue, reduced exercise capacity and recovery, through to suppressed immune systems and elevated risks of infections. Therefore, appropriate strategies to rapidly and safely synchronize circadian rhythms can help to mitigate the effects of travel.
The Physiology of Sleep
Similar to fatigue being a multi-factorial phenomenon, sleep is not triggered by a single hormone or mechanism, but rather by a host of physiological and environmental triggers.
Most adult humans require approximately 6-10 hours of sleep daily for proper functioning and health, though the primary purpose of sleep remains open to speculation. The brain appears to be the major control site for sleep regulation, specifically the supra-chiasmatic nucleus (SCN). The SCN is close to the primary temperature regulation centre in the hypothalamus, and also receives direct input from the eyes through a nerve running from the eyes to the hypothalamus. This anatomical location may explain why both temperature and light plays such a strong role in the setting and maintenance of circadian rhythms.
Sleep Deprivation and Exercise
The first obvious question is whether sleep deprivation has a practically relevant effect on overall exercise performance in field settings. Military training is one model for sleep deprivation research, especially with sustained operations training performed by special forces reducing sleep to minimal levels for days or weeks on end. While such events are excellent applications for sleep deprivation research, using them as experimental models is difficult because of the intertwined nature of extreme and continuous exercise, nutritional deficits, and sleep deprivation. Therefore, they should be viewed as indirect approaches to studying sleep deprivation and its effects on exercise.
The capacity for tolerating short-term sleep deprivation is supported by results from 72 h of simulated sustained military operations, featuring severe daily caloric deficit (~1600 kCal) and sleep deprivation (2 h of sleep daily) (Nindl et al. 2002). Some laboratory-based performance measures, such as squat jump power, decreased over the 72 h simulation. However, no decrements were observed with bench press power. In contrast, operationally-relevant tasks such as marksmanship and grenade throw were not impaired. The overall duration of the simulation was only 72 h, and may have been too brief to maximize impairment from accumulated sleep debt.
In a field study, both sleep deprivation (4 h of sleep daily) and caloric imbalance (-850 kCal) were milder, but the duration greatly prolonged to the full 61 days of real-life US Army Ranger training in a group of qualified military candidates (Young et al. 1998). All eight subjects successfully completed the full training program, as do a significant percentage of candidates for such special forces, demonstrating that prolonged exercise capacity can be maintained even in extreme situations of sleep and energy deficit for many fit and well-trained individuals.
A more direct study on exercise capacity and the physiological strain across multiple systems from adventure racing was provided by Lucas et al. (2008). During the 2003 Southern Traverse adventure race in New Zealand, competitors demonstrated a significant dropoff in self-paced exercise intensity, from an average of 64% HRmax in the first 12 h of the race down to an average of 41% HRmax from 24 h through the entire race. This was true in male and female members of both the winning and the last-place team, suggesting that the down-regulation was not a byproduct of race placement.
It can be argued that this decrease was due to general pacing errors, as an overly high initial pacing is common to most time trials of any distance. In support of this contention, heart rate responses did not differ during laboratory exercise tests performed pre- and post-competition, suggesting no autonomic impairment in cardiovascular regulation. However, subjective ratings of exercise intensity was significantly higher post-competition, likely due to changes in psychological motivation following such a strenuous competition. While no laboratory tests were performed, core temperature during competition remained within normal values despite wide-ranging environmental conditions.
Immediately following this competition, risks of upper respiratory and skin wounds, along with gastrointestinal problems, were elevated and fairly common. In addition, mood alterations were common, but this and the physical symptoms generally resolved themselves within two weeks.
In summary, it appears that, at least in highly motivated and trained individuals, ultra-endurance exercise and sustained operations can be tolerated with minimal decrements in exercise and operational capacity, and that any physiological or psychological impairments resolve relatively rapidly with appropriate recovery. One issue that must be raised, though, is the level of motivation and psychological characteristics of subjects in such environments, such that extrapolation to populations with lower inherent motivation may not be valid.
So in the end, while it can apparently be done by very, very psychologically motivated athletes, the best path to peak exercise capacity for most athletes remain ensuring adequate sleep every day and consistently throughout the week. Next time, we’ll look at select sleeping and alertness aids.
Lucas, S.J.E. et al. Intensity and physiological strain of competitive ultra-endurance exercise in humans. J Sports Sci 26:477-489, 2008.
Nindl, B.C. et al. Physical performance responses during 72 h of military operational stress. Med Sci Sports Exerc 34:1814-1822, 2002.
Reilly, T. and B. Edwards. Altered sleep-wake cycles and physical performance in athletes. Physiol Behav 90:274-284, 2007.
Young, A.J. et al. Exertional fatigue, sleep loss, and negative energy balance increases susceptibility to hypothermia. J Appl Physiol 85:1210-1217, 1998.
Stephen Cheung is a Canada Research Chair at Brock University, with a research specialization in the effects of thermal stress on human physiology and performance. He can be reached for comments at firstname.lastname@example.org .