Sleepiness during night-shift - sleeping habits or melatonin rhythm? A laboratory study

This study examined the relation between individual sleeping habits, and se nsitivity to sleepiness during five simulated night-shifts. Seventeen male volunteers were selected by their features of "morningness-eveningness" and flexibility or rigidity of sleeping habits. Subjective scales of sleepines s and fatigue, mental performance, sleep, attenuation alpha test and the me latonin circadian rhythm were measured. Data analysis compared the followin g groups: evening-morning flexible-rigid, evening-flexible, morning-flexibl e and morning-rigid groups, and two other definite groups from the total sa mple: a first group with an advanced onset of melatonin production at 23:00 and a second group (delayed onset) at 02.00. Results showed that morning, morning-flexible, morning-rigid and advance groups presented more adjustmen t difficulties than evening, evening-flexible and delay groups. Differences between flexible and rigid subjects remained less pronounced because of th e heterogeneity of samples. The results of this preliminary study suggest t hat melatonin production and a combination of the two dimensions "eveningne ss" and "flexibility" of sleeping habits could explain the capacity to adju st to night-work.