Although subjects were shielded from time cues (like clocks) and daylight, the researchers were not aware of the phase-delaying effects of indoor electric lights.[6] The subjects were allowed to turn on light when they were awake and to turn it off when they wanted to sleep. Electric light in the evening delayed their circadian phase; these results became well known.[7] More recent research has shown that adults have a built-in day, which averages about 24 hours; indoor lighting does affect circadian rhythms; and most people attain their best-quality sleep during their chronotype-determined sleep periods. A study by Czeisler et al. at Harvard found the range for normal, healthy adults of all ages to be quite narrow:[8] 24 hours and 11 minutes ± 16 minutes.