[Austin Both is the Associate Performance Director of Athletic Lab] In last week's installment, we discussed the effects of cognition and physical capacity as a result of sleep deprivation. Perhaps, even more important than these aforementioned factors, is sleep's ability to keep us alive (quite literally). While there is not a single unanimous explanation as to why sleep is required for life, after tireless research, we do have a greater understanding of the physiological functions that make it so important (Harvard.edu). One such mechanism that affects cognition, mood, as well as our physical state, is the tendency for sleep deprivation to cause an increase in cortisol levels (INSEP, Hausswirth, Mujika, p.102). While natural fluctuations of cortisol are a natural and healthy response to stress, physical or otherwise, chronically elevated levels of cortisol can cause a host of negative effects. Sleep deprivation's effect on cortisol levels is likely less than clinical cases. However, it can still cause high blood pressure, weakness, as well as sub-optimal metabolism of carbohydrates leading to weight gain (Saladin, p. 673). Coupling with increased cortisol levels, disruption of the Autonomic Nervous System (ANS) can create even more issues in terms of energy and mood. The ANS often goes under the radar because we have no volitional control over its constituents; however, its control reaches numerous systems, including the endocrine system and cardiac muscle (Saladin, p.572). Lack of sleep can cause chronic hyperactivity of the sympathetic division of the ANS (INSEP, Hausswirth, Mujika, p.102), which is a state associated with excitation. As a result, this state will trigger consistently elevated heart rate and respiration (Saladin, p. 443.) This results in a dangerous duo with the aforementioned increased cortisol levels. This elevated sympathetic activity [...]