Circadian adjustment of men and women to night work.

OBJECTIVES: The aims of the present study were to define the adjustment of sleep-wakefulness and other circadian rhythms during consecutive night shifts and to study the effect of gender on the adjustment to night work. METHODS: Twenty experienced shift workers, 9 men and 11 women, were studied under controlled laboratory conditions. After a night of habituation, the subjects worked one day shift and three night shifts. Rectal temperature was measured continuously during the study. Salivary melatonin and cortisol were assessed at 2-h intervals, and subjective sleepiness was checked every hour during the work shifts. Sleep was registered by the static-charge sensitive bed (SCSB) method after the workshifts. RESULTS: Subjective sleepiness varied both between the work shifts and between the men and women. The subjects felt most alert during the day shift and were the sleepiest during the first night shift. Sleepiness decreased during the consecutive night shifts but did not reach the level of the day shift during the three nights. The sleepiness of the women decreased more than that of the men, the difference being significant during the second and third night shifts (P < 0.05). The circadian rhythm of body temperature, salivary cortisol, and time in bed changed significantly (P < 0.05) between the workshifts, but no differences were found between the men and women in the circadian adjustment of the physiological variables to night work. CONCLUSIONS: The men felt sleepier than the women during the consecutive night shifts, although no physiological differences were found between the genders. However, since normal social and domestic factors were excluded, the findings cannot be considered as evidence of women's better adaptation to night work in everyday life.

Changes in maximal cardiorespiratory capacity among aging municipal employees.

Before and after a four-year follow-up, maximal oxygen consumption (VO2max) was determined for 67 municipal employees (initial age about 51 years). The VO2max (ml.kg-1) decreased 6% among the women and increased 8% among the men, the greatest and smallest change occurring for employees doing mental and physical work, respectively. The men's increase was associated with an increase in leisure-time physical activity. Work content was not associated with the level of VO2max for either gender. After the four years the women's absolute VO2max was about 66% of the men's. There were marked changes in VO2max at the individual level, the most marked decreases being associated with the use of medication in some cases. After the four years the women's VO2max of 63% was insufficient according to current recommendations. Therefore physically demanding jobs are not generally recommended for older women.

Interaction of age with shift-related sleep-wakefulness, sleepiness, performance, and social life.

It is not clear how the age-related changes in sleep are related to performance and subjective sleepiness at different time of the day. The aim of the present study was to study work shift related interactions of age with sleep-wakefulness, performance, and social life. A representative sample of aircraft maintenance workers in a continuous three-shift system was studied by a questionnaire (n = 275) and an on-site field (n = 49) study. In the field study, sleep length and quality and different ratings of social and other activities were studied with an actigraphy and a Pocket PC diary during 15 consecutive days. Subjective sleepiness (KSS) and vigilance performance (PVT) were registered at work. Although the shift type influenced the sleep, subjective sleepiness, performance, and social life, age was distinctly related only to shift-related changes in the amount of sleep, subjective sleepiness, and psychomotor vigilance. Night shifts were related with shorter sleep, decreased performance, and increased sleepiness. Although subjective sleepiness was greatest among the youngest (25-34 years) age group during the morning and the night shifts, the increase of performance lapses was higher among the middle-aged (35-49 years) and senior (50-58 years) groups during the night shifts compared to the youngest age group. According to the questionnaire, older shiftworkers also tended to perceive more frequently that subjective sleepiness decreases their work performance during the morning and night shifts. The results indicate of no direct link between age-related differences in subjective sleepiness and performance at night work. The shorter day sleep after the night shifts and higher deterioration of subjective and objective performance according to age urge on development of shift schedules aiming at lower fatigue levels during the night shifts.

Evaluation of a fast forward rotating shift schedule in the steel industry with a special focus on ageing and sleep.

The aim of this study was to explore the effect of a change in the speed and direction of shift rotation on the sleep and wakefulness of younger and older workers. A continuous three-shift schedule was changed from a slow backward rotating (EEE-MMMNNN-----) to a fast forward rotating system (MMEENN----). Sixteen subjects (mean age 42 years) were studied before and one year after the change in schedule. Two age groups were compared: ten younger men (mean age 35 years) and six older men (mean age 53 years). The effects of the new work schedule were evaluated by a questionnaire (modified SSI), and on-site registrations with an actigraph and sleep log for one shift cycle (10-15 days) before and after the new schedule. After the change in schedule, subjective sleep problems decreased and alertness increased during the morning shifts. The change in schedule influenced sleep differently in the two age-groups. Both the subjective and objective quality of sleep improved among the older workers. The results indicate that a fast forward rotating shift schedule is more suitable for older workers than a slower backward rotating system.

Physical work and strain involved in manual sorting of postal parcels.

A field study was conducted to assess cardiorespiratory and musculoskeletal stress and strain and work output during manual sorting of postal parcels, and to detect the effects of parcel sorting on the maximal muscle strength and endurance. The volunteer subjects comprised 32 healthy male sorters with mean (+/- s.d.) age of 34 +/- 7 years at five different sorting sites. Each subject was studied during one evening work shift. During the shift of 391 +/- 46 min the subjects manually sorted 1173 +/- 630 parcels and walked 4.7 +/- 2.3 km with and without the load. While sorting, heart rate was 101 +/- 18 beats min-1. In the heaviest tasks the oxygen consumption was 1.2 +/- 0.41 min-1, and no elevated blood lactate concentrations were found. Work postures in which the back was bent forward averaged 24% of the time for sorting. The overall cardiorespiratory rating and local ratings of perceived exertion for arms, back, and legs did not exceed the 'somewhat strong' level during the work shift. The maximal static strength both for the right and left hand-grip muscles was, on average, 3% lower (p less than 0.05) after the work shift than before the shift. No significant differences were found in the static or dynamic endurance times for the hand-grip muscles when the results obtained after the work shift were compared to the baseline values. At sorting centres the stress and strain on the cardiorespiratory and musculoskeletal system was evaluated to remain within acceptable limits for healthy male sorters.

Cardiorespiratory strain during walking in snow with boots of different weights.

In order to assess the physiological strain of different boot weights, seven male and three female subjects walked on a treadmill and a snow-field while wearing three types of boots: winter jogging boots (WJB), rubber boots (RB), and rubber safety boots (RSB), weighing (means +/- s.d.) 0.9 +/- 0.1, 1.9 +/- 0.4 and 2.5 +/- 0.2 kg, respectively During each walk the subjects wore the same clothing ensembles and moved at the same, individually determined speed. The mean (+/- s.e.) depths of the footprint impression in the snow while walking in the WJB, RB, and RSB were 26.1 +/- 1.5, 25.6 +/- 1.4 and 26.1 +/- 1.5 cm (NS), respectively. During walking on the treadmill, the means for oxygen consumption were 0.79 +/- 0.05, 0.81 +/- 0.06 and 0.83 +/- 0.04 l min-1 (NS) and in snow 2.24 +/- 0.18, 2.34 +/- 0.17 and 2.34 +/- 0.19 l min-1 (p less than 0.01) with the WJB, RB and RSB, respectively. The mean oxygen consumption levels observed during the walks averaged 23% and 65% of the subject's maximum oxygen consumption on the treadmill and in the snow-field, respectively. During the walking tests the corresponding mean heart rates were 106 +/- 4, 93 +/- 5, and 95 +/- 5 beats min-1 (p less than 0.05) on the treadmill, and 151 +/- 11, 150 +/- 11 and 151 +/- 12 beats min-1 (NS) in snow. No significant differences in ratings of perceived exertion were observed between the walking tests in snow with the three types of boots. In accordance with earlier studies, walking in snow was found to be strenuous work. In conclusion, the use of the RSB is recommended during logging work in snow, since they are known to provide greater protection than lighter boots and the increase in physiological strain experienced with RSB in this study was not appreciably greater than that with boots of lighter weight.

Age and adjustment to night work.

OBJECTIVE: The study was designed to examine the effects of age on sleep and the circadian rhythms during consecutive night shifts. METHODS: Two groups of letter sorters (19-29 (n = 7) and 53-59 (n = 7) years of age were studied in a sleep laboratory under closely controlled conditions. After two baseline days, circadian adjustment to three night shifts was monitored by continuous measurement of rectal temperature, salivary melatonin, and sleep-wakefulness during the night shifts. RESULTS: Age was significantly related to the adjustment to night work of rectal temperature minimum and of self rated sleepiness. Young subjects delayed their temperature phase and decreased sleepiness more than the older subjects. Age was also significantly related to an increase of alertness and to the feeling of being refreshed after the day sleep. Although there were basic differences in sleep duration and structure between the age groups, the latter did not change across the night shifts. CONCLUSION: Aging decreases the ability to recover after several, but not after the first night shift.