Normal Circadian Rhythms

Among people with healthy circadian clocks, there are "larks" or "morning people" who prefer to sleep and wake early, and there are "owls" who prefer to sleep and wake at late times. But whether they are larks or owls, people with normal circadian systems:

• can wake in time for what they need to do in the morning, and fall asleep at night in time to get enough sleep before having to get up.
• can sleep and wake up at the same time every day, if they want to.
• will, after starting a new routine which requires they get up earlier than usual, start to fall asleep at night earlier too within a few days. For example, someone who is used to sleeping at 1 am and waking up at 9 am begins a new job on a Monday, and must get up at 6 am to get ready for work. By the following Friday, the person has begun to fall asleep at around 10 pm, and can wake up at 6 am and feel well-rested. This adaptation to earlier sleep/wake times is known as advancing the sleep phase, and healthy people can advance their sleep phase by about one hour each day.

Researchers have placed volunteers in caves or special apartments for several weeks without clocks or other time cues. Without time cues, the volunteers tended to go to bed an hour later and to get up about an hour later each day. These experiments demonstrate that the "free-running" circadian rhythm in humans is about 25 hours long. To maintain a 24 hour day/night cycle, the biological clock needs regular environmental time cues, e.g. sunrise, sunset, and daily routine. Time cues are what keep our body clocks aligned with the rest of the world.

Biological Timing Tutorial from the National Science Foundation Center for Biological Timing

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