Experimental schedule. Subjects arrived at the sleep laboratory around 21:00 and were instructed to go to bed at 00:00 (lights off) after preperations were made to measure PSG and NIRS. Experiments under each of the two conditions were conducted twice for each subject in a consecutive two-night, single-blind crossover setting (“request” and “surprise”) without notice of which condition would be examined first. Each subject was randomly allocated to one of two conditions and instructed at bedtime to wake up at: 1) 03:00 (“request”); or 2) 08:00, but was unexpectedly woken at 03:00 (“surprise”). When the subject could self-awaken at 03:00 ± 30 min, the result was recorded as “success” or otherwise as “failure”. Each subject was examined under the other condition the following day.