The people studied by the researchers………. .

Sleep researchers have found that people can make themselves wake up at a given time simply by deciding to do so before they go to sleep. Scientists took two groups of volunteers and, at nightfall, told one group that they would be woken at 6 a.m. and the other that they would be woken at 9 a.m. The sleepers' levels of the hormone adrenocorticotropin, which is known to cause spontaneous awakening, were then measured. In each group, there was a rise in the levels of the hormone one hour before the volunteers expected to get up. The three-hour difference between the rise in hormones in the two groups suggests that the body can be programmed to wake up on command.