Free-running Sleep

Free-running sleep experiments can involve any organism which sleeps. Free-running sleep is sleep which is not adjusted, entrained, to the 24-hour cycle in nature nor to any artificial cycle. Such experiments are used in the study of circadian and other rhythms in biology. Subjects are shielded from all time cues, often by a constant light protocol, by a constant dark protocol or by the use of light/dark conditions to which the organism cannot entrain such as the ultrashort protocol of one hour dark and two hours light. Also, limited amounts of food can be made available at short intervals so as to avoid entrainment to mealtimes. Subjects are thus forced to live by their internal circadian "clocks".

The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of gene-activity, body temperature, blood pressure, hormone secretion and/or sleep and activity/alertness. Alertness in humans can be determined by many kinds of verbal and non-verbal tests; activity in animals by observation, for example of wheel-running in rodents.

When animals or people free-run, experiments can be done to see what sort of signals, known as zeitgebers, are effective in entrainment. Also, much work has been done to see how long or short a circadian cycle the different organisms can be entrained to. For example, some animals can be entrained to a 22-hour day, but they can not be entrained to a 20-hour day. In recent studies funded by the U.S. space industry, it has been shown that most humans can be entrained to a 23.5-hour day and to a 24.65-hour day.

The effect of unintended time cues is called masking. If morning rush traffic can be heard from outside, if researchers or maintenance staff appear at the same time each day, an experiment can be ruined by masking.