The “Forbidden Zone for Sleep” might be caused by the evening thyrotropin surge and its biological purpose is to enhance survival: a hypothesis

It is possible for normal subjects to fall asleep during daytime wakefulness, and the propensity for daytime sleep varies being high in the mid-afternoon and lower in the early evening 2 or 3 hours before overnight sleep onset. This lowering of propensity for sleep in the few hours that precede overnight sleep is a paradoxical phenomenon termed the “Forbidden Zone for Sleep” or “Wake Maintenance Zone”. During the day, thyrotropin levels, which reflect the activity of the hypothalamus-pituitary-thyroid axis, oscillate in a predictable rhythm: lower levels during afternoon and higher levels during the early evening Forbidden Zone for Sleep. The steep increase in thyrotropin levels in the early evening is called the “thyrotropin-evening-surge”, with the higher levels just before overnight sleep onset. Thereafter thyrotropin levels decrease over the rest of the night reaching their low daytime levels. Considering the well-known role of the thyroid in vigilance and alertness, we propose that the negative correlation between thyrotropin levels and propensity to sleep is not a coincidence. Assuming that the circadian rhythms of any species are turned to facilitate survival, we propose that the Forbidden Zone for Sleep is an inherent part of the human circadian rhythm increases biological fitness. In this article, a medical hypothesis, we present evidence that in human’s circadian rhythms the Forbidden Zone for Sleep is secondary to the thyrotropin evening surge.