Closely monitored repeated tasks 

The subjects mainly stayed in their room for the whole 42 hours, and were constantly kept active to prevent them from falling asleep. During the experiment, they carried out repeated tasks 13 times during functional magnetic resonance imaging (fMRI) sessions. "While the participants were in their room", Pierre Maquet continues, "we repeated another series of tasks every hour, including a motor inhibition test. Successive figures appeared on the screen. According to the figure, the subjects had to press, or not press, on a button. Deprived of sleep, they experienced greater difficulty at inhibiting this movement, or on the contrary, their drowsiness prevented them pressing on the button for any of the figures. During the fMRI, they also took a series of tests that quantified their state of vigilance and their working memory. For instance, they were presented with a timer set at zero on the screen. As soon as it started running up the milliseconds, the subjects had to stop it as quickly as possible. During an auditory test, they heard a sequence of letters which they then had to tell us about. Was one letter the same as the one heard three items earlier? Etc." These sessions lasted about 10 minutes and required the subjects sustained attention. Maintaining such a level of vigilance is clearly difficult for someone who has been awake for more than 40 hours. But it is exactly these changes that help to quantify the effects of the circadian cycle and sleep deprivation.  

Secretion of melatonin and sleep deprivation

The data collected was transferred to a timescale. The quality of the execution of the various tasks showed similar patterns that weren't in the least monotonic. "During the day, performance remained quite stable, but ultimately declined owing to sleep deprivation. We observed a sudden increase in reaction time after 22:00, a time that corresponds to the moment when the body starts to secrete melatonin." 

The graph on the left illustrates the evolution in performance relating to the task measuring the reaction time. The grey curve corresponds to the melatonin secretion cycle. The hours are expressed in circadian terms (at the bottom of the x-axis), opposite the corresponding clock time (at the top of the x-axis). Time 0, corresponding to 22:00, indicates the beginning of melatonin secretion. The graph on the right illustrates subjective somnolence. The subjects had to assess their own vigilance on a scale of zero to nine. Zero referred to perfect vigilance and nine to the highest level of somnolence. The evolution of the two curves follows the secretion of melatonin, which falls slightly at the end of the first night to reach a stable level until dusk on the second day. 

During the night, performance declines until early morning before improving, despite continued sleep deprivation. "Performance isn't as good as on the first day, which is normal", the researcher adds. "It's affected by sleep deprivation. However, it doesn't collapse in a linear manner. When the melatonin is inhibited again, the subjects recover part of their faculties for the day, before reaching a new and even bigger peak on the second evening. This data quite clearly shows an addition of incidences linked to the circadian rhythm on the one hand, and sleep deprivation on the other."