The role of thyroid hormone in sleep deprivation.

Sleep deprivation is a stressful condition, as the subject experiences feelings of inadequate well-being and exhibits impairments in his/her functioning. However, in some circumstances sleep deprivation may be crucial for survival of the individual. Most likely, complex neural circuits and hormones play a role in allowing sleep deprivation to occur. For instance, thyroid hormone activity sharply increases when an individual is in a state of sleep deprivation. We believe that this increase is central to sleep deprivation physiology. During sleep deprivation, the hypothalamic-pituitary-thyroid axis initially increases as a consequence of increased release of thyroid stimulating hormone from the pituitary. Subsequently, as sleep deprivation continues, the sympathetic nervous system is recruited through its anatomical connection with the thyroid gland. While thyroid stimulating hormone levels markedly increase during sleep deprivation, it has been suggested that these increases are secondary to sleep deprivation. However, there is little evidence to support this assumption. We believe that the physiology of the thyroid axis during sleep deprivation and the actions of the effector hormone thyroid hormone suggest that thyroid hormone inhibits sleep and not the contrary. To our knowledge, few studies have addressed the possible neural functions that enable sleep deprivation. In this article, we discuss the hypothesis that an augmentation in the thyroid hormone axis is central to a subject's ability to curtail sleep.