coffee disliked the taste so much on a mission that he stopped drinking it after returning to earth); some astronauts enjoy eating certain foods that they would not normally eat, and some experience no change whatsoever. multiple tests have not identified the cause, and several theories have been suggested, including food degradation, and psychological changes such as boredom. astronauts often choose strong-tasting food to combat the loss of taste. additional physiological effects within one month the human skeleton fully extends in weightlessness, causing height to increase by an inch. after two months, calluses on the bottoms of feet molt and fall off from lack of use, leaving soft new skin. tops of feet become, by contrast, raw and painfully sensitive, as they rub against the handrails feet are hooked into for stability. tears cannot be shed while crying, as they stick together into a ball. in microgravity odors quickly permeate the environment, and nasa found in a test that the smell of cream sherry triggered the gag reflex. various other physical discomforts such as back and abdominal pain are common because of the readjustment to gravity, where in space there was no gravity and these muscles could freely stretch. these may be part of the asthenization syndrome reported by cosmonauts living in space over an extended period of time, but regarded as anecdotal by astronauts. fatigue, listlessness, and psychosomatic worries are also part of the syndrome. the data is inconclusive; however, the syndrome does appear to exist as a manifestation of the internal and external stress crews in space must face. psychological effects studies of russian cosmonauts, such as those on mir, provide data on the long-term effects of space on the human body. research the psychological effects of living in space have not been clearly analyzed but analogies on earth do exist, such as arctic research stations and submarines. the enormous stress on the crew, coupled with the body adapting to other environmental changes, can result in anxiety, insomnia and depression. stress there has been considerable evidence that psychosocial stressors are among the most important impediments to optimal crew morale and performance. cosmonaut valery ryumin, twice hero of the soviet union, quotes this passage from "the handbook of hymen" by o. henry in his autobiographical book about the salyut 6 mission: "if you want to instigate the art of manslaughter just shut two men up in an eighteen by twenty-foot cabin for a month. human nature won't stand it." nasa's interest in psychological stress caused by space travel, initially studied when their crewed missions began, was rekindled when astronauts joined cosmonauts on the russian space station mir. common sources of stress in early american missions included maintaining high performance while under public scrutiny, as well as isolation from peers and family. on the iss, the latter is still often a cause of stress, such as when nasa astronaut daniel tani's mother died in a car accident, and when michael fincke was forced to miss the birth of his second child. sleep main article: sleep in space the amount and quality of sleep experienced in space is poor due to highly variable light and dark cycles on flight decks and poor illumination during daytime hours in the spacecraft. even the habit of looking out of the window before retiring can send the wrong messages to the brain, resulting in poor sleep patterns. these disturbances in circadian rhythm have profound effects on the neurobehavioural responses of the crew and aggravate the psychological stresses they already experience. sleep is disturbed on the iss regularly due to mission demands, such as the scheduling of incoming or departing space vehicles. sound levels in the station are unavoidably high because the atmosphere is unable to thermosiphon; fans are required at all times to allow processing of the atmosphere, which would stagnate in the freefall (zero-g) environment. fifty percent of space shuttle astronauts took sleeping pills and still got 2 hours less sleep each night in space than they did on the ground. nasa is researching two areas which may provide the keys to a better night's sleep, as improved sleep decreases fatigue and increases daytime productivity. a variety of methods for combating this phenomenon are constantly under discussion. duration of space travel a study of the longest spaceflight concluded that the first three weeks represent a critical period where attention is adversely affected because of the demand to adjust to the extreme change of environment. while skylab's three crews remained in space 1, 2, and 3 months respectively, long-term crews on salyut 6, salyut 7, and the iss remain about 5–6 months, while mir expeditions often lasted longer. the iss working environment includes further stress caused by living and working in cramped conditions with people from very different cultures who speak different languages. first-generation space stations had crews who spoke a single language, while 2nd and 3rd generation stations have crews from many cultures who speak many languages. the iss is unique because visitors are not classed automatically into 'host' or 'guest' categories as with previous stations and spacecraft, and may not suffer from feelings of isolation in the same way. future use space colonization efforts must take into account the effects of space on the human body. the sum of human experience has resulted in the accumulation of 58 solar years in space and a much better understanding of how the human body adapts. in the future, industrialisation of space and exploration of inner and outer planets will require humans to endure longer and longer periods in space. the majority of current data comes from missions of short duration and so some of the long-term physiological effects of living in space are still unknown. a round trip to mars with current technology is estimated to involve at least 18 months in transit alone. knowing how the human body reacts to such time periods in space is a vital part of the preparation for such journeys. on-board medical facilities need to be adequate for coping with any type of trauma or emergency as well as contain a huge variety of diagnostic and medical instruments in order to keep a crew healthy over a long period of time, as these will be the only facilities available on board a spacecraft for coping not only with trauma but also with the adaptive responses of the human body in space. at the moment only rigorously tested humans have experienced the conditions of space. if off-world colonization someday begins, many types of people will be exposed to these dangers, and the effects on the very young are completely unknown. on october 29, 1998, john glenn, one of the original mercury 7, returned to space at the age of 77. his space flight, which lasted 9 days, provided nasa with important information about the effects of space flight on older people. factors such as nutritional requirements and physical environments which have so far not been examined will become important. overall, there is little data on the manifold effects of living in space, and this makes attempts toward mitigating the risks during a lengthy space habitation difficult. testbeds such as the iss are currently being utilized to research some of these risks. the environment of space is still largely unknown, and there will likely be as-yet-unknown hazards. meanwhile, future technologies such as artificial gravity and more complex bioregenerative life support systems may someday be capable of mitigating some risks.