Pandi Perumal and Gonfalone presented perspective on sleep

Pandi-Perumal and Gonfalone presented perspective on sleep in space by overviewing the past medical reports . The authors made special emphasis on “normal sleep” in space, where no gravity exists. According to their review, sleep in space was mainly monitored by subjective sleep diaries and actigraphy (wrist-worn accelerometer). As sleep cycle was developed in response to the 24h rotation of the earth, and refreshment by sleep seems not satisfactory, which would lead to adverse health effect. There is a difference between Bindarit Supplier activity and physical movement during sleep, and the discrepancy of sleep parameters between polysomnography and actigraphy is obvious for insomniacs . I want to make caution on sleep monitoring method, especially on actigraphy. Barger and colleagues clarified the change of sleep problems in astronauts including period of spaceflights, and recognized sleep deficiency before spaceflight by actigraphy . As they conducted their study in the earth and space with different level of gravity, validation study of actigraphy against sleep polysomnography in a weightless environment is needed. In addition, Barger and colleagues presented the difference of total sleep time by subjective sleep diaries and actigraphy , which could be observed during spaceflight and in the earth. There is no evidence on the superiority of objective sleep parameter against subjective evaluation, and the methodological validity on actigraphy is important for the evaluation of sleep deficiency during spaceflight. Finally, I suppose that sleep medication is sometimes needed for keeping good sleep in astronauts. Validation study of actigraphy is also needed for this situation . Conflict of interest
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Introduction Several functional roles are attributed to sleep. One view is that sleep is required for energy conservation and nervous system recuperation [1]. Even though glucose consumption is reduced during the slow wave activity (SWA) phase of sleep relative to the wake state [3], a certain amount of energy is consumed for synaptic activity during sleep [2]. Cellular energetics during sleep are found to be associated with metabolic function and gene transcription [4] and sleep promotes mRNA translation for protein synthesis [5]. However, there are no causal factors indicating the need for sleep either for gene expression or metabolic activity. Even though unwanted metabolic products from the adult brain are cleared during sleep [6], there are no indications why this function requires a reduction in the level of consciousness. Other systems in the body that operate throughout the life of animals, such as the heart and kidneys, do not shut down their major functions at any time. In contrast, the nervous system alters the level of consciousness during sleep, putting the animal at risk for attack by its predators. This leads to the question – What specific function of the nervous system necessitates altering its basic function of maintaining the state of consciousness, a necessary background state required for carrying out other higher brain functions? Sleep is thought to facilitate learning and memory [7], memory consolidation [8], consolidation of cortical plasticity [9] and cortical response potentiation following visual experience [10]. However, these experimental findings are not sufficient to provide an adequate explanation for the state of unconsciousness associated with sleep [11]. The observation that mutation in a transcriptional repressor reduces sleep by two hours every day [12] has not yet provided an explanation for the necessity for sleep. In this context, different proposals for possible mechanisms for sleep were put forward [13,14]. Even though energy conservation, rejuvenation and facilitation of learning and memory can be contributing factors, a mechanism that results in losing consciousness that puts the animal at severe risk for survival is still lacking. This naturally raises the question – Why didn’t evolution eliminate the need for sleep that would have enabled animals to remain conscious throughout the twenty-four-hour period? Conservation of sleep indicates that sleep has an indispensable role in nervous system operations. Sleep-like states observed in lower-level animal species such as flies and worms [15–17] indicate a universal functional role for sleep.