The functions of sleep go far beyond recuperation. Among others, sleep impacts metabolism, immune function, memory consolidation, reaction time, decision making, psychological well-being, mental state and eating behaviour. Thus, sleep quality and quantity are clearly among the most important determinants for health, and conversely, sleep problems, disturbances and pathologies represent major risk factors for disease and disability.
Sleep timing is regulated by the body clock, which is synchronised by light and darkness. This ‘entrainment’ differs between individuals (the proverbial ‘larks’ and ‘owls’; chronotypes). Modern life-styles weaken our clocks’ synchronisers and have drastically changed our sleep patterns with significant consequences – the direct and indirect costs of sleep-related problems are estimated to approach 1% of the gross domestic product. Yet, we still know surprisingly little about this important state of our lives.
Research has made great advances in understanding neurotransmitters and brain regions involved in sleep and how the timing of sleep and wakefulness are controlled by the circadian clock. Yet, we still lack basic information about sleep’s fundamental position in our existence. One reason for this lack of knowledge is that most of what is known comes from sleep being investigated in laboratories with subjects who have been instructed to sleep at certain times in beds that are not their own, with electrodes fastened to their heads. In addition, assessments of sleep quality, which is central for quantifying the effects of sleep on health, are often based on subjective responses to questions about how ‘well’ people feel at wake-up or whether they think they experienced a good night’s sleep.
To answer these basic questions about sleep, we have to learn more about sleep in its context – in the real world (in situ) with the help of a Human Sleep Project (HSP). We have to collect sleep- relevant data world-wide, comparing different geographical locations, different seasons and climates, cultures, and states of industrialisation/electrification. First results of this endeavour show the power of this approach.