Work at sea: a study of sleep, and of circadian rhythms in physiological and psychological functions, in watchkeepers on merchant vessels

Abstract

Daily diary records of sleep and activity, and 4-h measurements of body temperature, performance and subjective alertness were collected on board ship from 15 watchkeepers on the 4-on/8-off system, and from 28 dayworkers, on both westward and eastward transatlantic voyages. The data from a balanced sample of the subjects were analysed over selected 8-d periods of the voyages where four or five time zones were crossed. During these periods the average amount of daily sleep obtained by dayworkers on the eastward voyage was more than 1 h less than that on the westward voyage, and its quality was rated lower. Watchkeepers' main sleep was also shorter when travelling eastward, but this reduction was partially compensated for by a slightly longer secondary sleep. With the exception of subjective alertness on the eastward voyage, the basic phase of the circadian rhythms in the measured variables adjusted appropriately to the clock changes associated with the time zone crossings. The normal shape of the average daily curves was, however, altered differentially in the two directions of travel; as a result, morning levels of all variables were lower on the eastward voyage than on the westward, but evening levels were higher. These distortions of rhythm waveforms, which probably arose from a combination of endogenous and exogenous factors, add another dimension to the basic problem caused by the effects of circadian rhythms on operational efficiency in the shipboard situation. This problem can only be solved by the development of alternative watchkeeping systems which take full account of these rhythms.

Circadian misalignment on submarines and other non-24-h environments – from research to application

“…The results showed that crew members using the former schedule but not the latter one exhibit superposition of 18-h and 24-h periods in all tested variables. Asimilar superposition was also observed in other studies [ 61 , 62 , 66 ]. As the schedule continues, the impact on circadian rhythms becomes more pronounced.…”

“…Naitoh et al [ 65 ] found that a 6-h on/12-h off routine during a nuclear submarine patrol caused a loss of 24-h rhythmicity in oral temperature, which was due not only to a decreased circadian amplitude but also to a dispersion of the time of the peak. A different watch time in three shift groups who used alternative 4-h on/8-h off regimens induced changes in the phase of rectal temperature, noradrenaline excretion, and adrenaline excretion [ 66 ].…”

“…Moreover, in a real mission, jet lag may further contribute to the effects of circadian desynchronization. Condon et al [ 66 ] found that the sleep length of watchkeepers was shorter when they crossed four to five time zones eastbound compared to crossing the time zones westbound, and the subjective alertness showed slower adaptation on eastbound travel. This finding may be explained by the fact that the human free-running period is ~ 25 h, which facilitates westbound adaptation.…”

The effect of circadian rhythms on shipping accidents

“…Compared with other industries, marine watchkeeping is influenced by many other factors, such as noise, motion, time zone crossings in navigations associated with rapid changes in longitude, and alterations in daylight length in navigations with significant changes in latitude. These peculiarities, which are inherent to maritime transport, disrupt the operational efficiency of watchkeepers (Condon et al, 1988).…”

Sleep deprivation and a non–24‐h working schedule lead to extensive alterations in physiology and behavior

“…A substantial proportion of people in various occupations undergo rotating working schedules, such as those working in logistics, the catering industry, and international cabin crews. Under certain special conditions, some people are also required to live and work in non-24-h working and rest schedules, such as the non-24-h working schedules with 8 h on and 4 h off (8 h for working and rest, and 4 h for sleep) or 12 h on and 6 h off (12 h for working and rest, and 6 h for sleep), which are prevalently used in maritime operations, oil mining, and some other industries (12)(13)(14)(15)(16)(17)(18)(19)(20). Split-duty schedules might be considered as an alternative to longterm shifting rosters, but the physiologic circadian and diurnal rhythms and, as a consequence, metabolism and performance are also subject to changes under the non-24-h schedules (21).…”