Local sleep-like events during wakefulness and longterm space travel

Abstract

Since the Moon race in the 60s, human kind has continuously been orbiting Earth. However, before we can envision a permanent settlement on the Moon and explore our solar system, human physiology adaptation challenges need to be answered. Adequate sleep quantity and quality are required for good cognitive performances and chronic sleep restriction comes with a high cost. Sleep quality and sleep quantity are regulated by two interacting processes, the homeostatic and the circadian process. Electroencephalographic (EEG) theta activity (5-7Hz) during wakefulness reflects both the homeostatic and the circadian process. The circadian influence can be seen by a diurnal oscillation of theta activity. The homeostatic component, on the other hand, builds up with the time awake. Theta activity is therefore considered a sleep pressure marker and was associated with alertness and subjective daytime sleepiness. Intracranial recordings in rats showed that an increase of sleep pressure was also linked to neuronal off periods during wakefulness, similar to what can be recorded during slow waves sleep. These local sleep-like events during wakefulness have also been studied in the human EEG as markers of sleep pressure and widespread local sleep-like events are thought to be responsible for the decrease in cognitive performances under high sleep pressure conditions. Sleep quality and quantity in space will be key for maintaining astronaut’s cognitive functions and improving missions’ success rate. Even though astronauts are allocated enough time to sleep, a decrease of sleep quantity was reported on the International Space Station (ISS). Furthermore, for proper dissipation of sleep pressure, the quality of sleep is also important. In space, sleep quality might be impacted by external factors such as microgravity, isolation, confinement, circadian misalignment, chronic stress, temperature, light and noise disturbances. Besides space missions, analogue missions are opportunities to study human acclimatisation to space-like environment. Concordia station in Antarctica is one of the most remote human outpost on Earth. Studies conducted in Antarctica suggested that isolation, high altitude and constant darkness might disturb sleep in a similar fashion as on the ISS. In this thesis we investigated sleep pressure markers on the ISS. Then, to disentangle the effects of space environmental factors, we studied sleep pressure markers during a space analogue mission on Earth. Additionally, we studied sleep pressure's impact on cognitive performances and we explored novel countermeasures to enable human deep space travel. In this regard, we subdivided our work into three lines of research with three associated research papers.