The study of human sleep has so far been possible only under controlled conditions in dedicated sleep laboratories. Thanks to the development of a new analytical technique, experimental subjects can sleep in their own beds while contributing to the scientific understanding of what remains a mysterious biological phenomenon. For a simple wrist-worn device now suffices to record the essential characteristics of natural sleep bouts. The technique is validated in a new paper published in the journal Current Biology, which reports work carried out under the direction of Professor Till Roenneberg, who heads the Human Chronobiology Group at the Institute of Medical Psychology at LMU, and Dr. Eva Winnebeck, who is in charge of its Sleep Laboratory.
Sleep is not a resting state in which we remain immobile. Instead it is characterized by phases of involuntary movement, which correspond to distinct physiological states. In the third phase of their long-term Human Sleep Project, the chronobiologists at LMU took advantage of this to study the dynamics of sleep phases in over 16,000 sleep bouts from 593 experimental subjects between the ages of 8 and 92. By focusing on phases of immobility and contrasting them with phases of movement during natural sleep, they were able to detect rhythms in movement that recurred at intervals of approximately 90 minutes. The study’s authors called their measure “Locomotor Inactivity During Sleep” (LIDS). The study confirms that the alternation of bursts of movement and bouts of immobility essentially correspond to the known cycles of REM and non-REM (deep) sleep. These phases are associated with the onset and cessation of rapid (involuntary) eye movements (REMs) which occur several times every night. However, while these sleep cycles of REM and non-REM sleep have so far been measured by means of electroencephalography in sleep laboratories, the pattern of LIDS can now be easily detected with wrist-worn monitors.
The editors of Current Biology have hailed this new approach as a “breakthrough” in sleep research. “Our new method makes it possible to collect objective information on the sleep characteristics and phases exhibited by individuals outside the usual laboratory setting, using a simple recording and analysis technique. With this approach the whole world becomes a sleep laboratory,” says Till Roenneberg. As a result, the scientists have already found that men move more during their sleep than women. Furthermore, shift workers, who frequently have to alter their sleeping times, also move more than day workers with regular sleeping times. And the younger one is, the more pronounced are the movement rhythms and the more one increases one’s movement towards the end of the sleep period.
The wristwatch-like device used is an ‘actimeter’, which can provide a continuous record of wrist locomotor activity over periods of up to several months. It works on the same principle as commercially available smart watches that detect changes in position and acceleration, which also try to monitor sleep phases. “Our method is however simple, transparent and designed specifically for long-term measurements,” Roenneberg explains. At all events, the validation of the new tool’s efficacy will make it easier for researchers to investigate the factors that affect the quality of natural sleep, which has hitherto been difficult to measure objectively. It may therefore someday be able to help people who suffer from insomnia and other sleep disturbances. “We also hope that it will make people more aware of the importance of sleep for our general health and wellbeing,” says Roenneberg.