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Sleep and Human Performance

Author(s): David F. Dinges, PhD

Human Genetic Molecular "Clock"

Slide Notes
Sleep is timed by a biological (genetic molecular) “clock” in the brain that is circadian, which means “about a day” or approximately 24 hours. This “clock” is responsible for our sleepiness and desire for sleep at night, during our natural sleep times, and it helps keep us asleep throughout the night. If we stay awake at night working or playing, our biological clock stills forces physiological changes on our brains and bodies during the night. These changes include loss of body heat via vasodilation (which results in our feeling cold), microsleeps that can lead to performance lapses, and feelings of fatigue and low mood. On the left hand of the slide, note the shorter period of daylight in the northern hemisphere (where it’s winter) and the correspondingly longer period of daylight in the southern hemisphere (where it’s summer). This occurs because Earth is tilted approximately 23º on its axis as it rotates around the sun. The key point is that sleep-wake cycles, and therefore, our alertness and performance, depend on both how much sleep we obtain and on our internal biological clocks, which anticipate Earth’s orbital mechanic (i.e., daily rotation on its axis) and responds (resets daily) to the light-dark cycle. It is the the slow adjustment of the internal circadian clock in our brains to new schedules that makes it so difficult for us to adjust to night shift work and to jet lag (i.e., flying across time zones), as well as to the switch from sleeping late in the summer, to getting up early in the school year. The circadian biological “clocks” in our brains define us as Earthlings. In contrast, life on another planet—like Mars—would likely have an internal biological clock that anticipated the Martian day, which is 24.67 hours. 

Transcript of Videotaped Presentation (
Our biological need for sleep emanates from structures in our brains, one of which is called the “biological clock”. This clock controls our tendency to want to sleep at night and to be awake in the day time. It also controls or modulates our performance. In fact, the connection between our biological clock and where we are located on Earth is fundamental. The biological clock in the human brain appears on the slide as red dots. These dots are receptors for a hormone called melatonin, which is secreted by the pineal gland in the brain. Melatonin is a light sensitive hormone that is suppressed by light in the day time and secreted at night in the dark. Our biological clock, the areas marked in red on the image of the human brain, is a real clock. It actually changes our physiology through a near-exact 24-hour cycle: 24 hours and a few minutes. And of course that is the period of Earth’s rotation on its axis. So as life developed on Earth, we find these biological clocks anticipate Earth’s rotation through the light and dark cycle. And some animals, including humans, that have clocks phased one way are diurnal (that is, awake in the day and asleep at night). Some animals are nocturnal; they’re awake at night and sleep during the day. They have a broad flat face and big eyes so they amplify light at night and hear well. And then many others are awake and asleep on the light/dark boundary. Of course the light period varies, based on where you are on the planet, and how far away from the equator you may be. It’s 12 hours equal at the equator. And that’s because Earth is tilted, about 23 degrees on its axis, as it rotates annually around our sun. So in the winter, the photo period, or the amount of light, is fairly short in the northern hemisphere, and is long in the southern hemisphere. These conditions are reversed in what we call summer and they call a winter in the southern hemisphere. The point is that the regulatory systems for sleep are very old biologically and are related to the actual timing of light/dark and energy availability on the planet.

Funded by the following grant(s)

National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.