Editing Free-running sleep (section)

==Background==
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 or [[sleep]] and activity/[[alertness]]. Alertness in humans can be determined by many kinds of verbal and non-verbal tests, whereas alertness in animals can usually be assessed by observing physical activity (for example, of wheel-running in [[rodent]]s).

When animals or people ''free-run'', experiments can be done to see what sort of signals, known as [[zeitgeber]]s, are effective in entrainment. Also, much work has been done to see how long or short a circadian cycle can be entrained to various organisms. 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.<ref>{{cite journal |last=Scheer |first=Frank A.J.L. |author2=Kenneth P. Wright Jr. |author3=Richard E. Kronauer |author4=Charles A. Czeisler |date=2007-08-08 |title=Plasticity of the Intrinsic Period of the Human Circadian Timing System |journal=PLOS ONE |volume=2 |pages=e721 |quote=[E]xposure to moderately bright light (~450 lux; ~1.2 W/m<sup>2</sup>) for the second or first half of the scheduled wake episode is effective for entraining individuals to the 24.65-h Martian sol and a 23.5-h day length, respectively. |doi=10.1371/journal.pone.0000721 |pmid=17684566 |issue=1 |pmc=1934931 |bibcode=2007PLoSO...2..721S |editor1-last=Nicolelis |editor1-first=Miguel|doi-access=free }}</ref>

The effect of unintended time cues is called ''masking'' and can totally confound experimental results. Examples of masking are morning rush traffic audible to the subjects, or researchers or maintenance staff visiting subjects on a regular schedule.