Permanent night work alters characteristics of circadian rhythm of rest-activity in human subjects

The objective of this study was to explore the impact of permanent night shift on rest-activity rhythm in 20 randomly selected male permanent night workers, employed in a local newspaper printing office. The machine operators performed their job from 20:00 to 04:00 and the computer operators from 18:00 to 02:00 every day. The night workers availed one-day rest after working six consecutive days. Twenty-one age- and gender-matched day workers were also studied for comparison of rhythm parameters. In each subject rest-activity rhythm was monitored non-invasively by using Actiwatch (AW64 Mini Mitter Co. Inc., USA). Data were gathered at one-minute intervals for seven consecutive days. In addition, all subjects also responded to the morningness-eveningness questionnaire (Horne and Ostberg's MEQ). Data were stored in the form of records and analyzed with the help of Actiware sleep software, Cosinor rhythmometry, Power spectrum analysis, Autocorrelation, ANOVA and Duncan's multiple range test, t-test and descriptive statistics. Rhythm parameters were computed at two different fixed windows, namely =24h and =12h. Harmonic means were calculated for those parameters obtained at both windows. All permanent night workers and day workers exhibited statistically significant circadian rhythm in rest-activity profile. At least five permanent night workers exhibited prominent non-circadian period. Statistically significant differences were observed between the rhythm characteristics of permanent night workers and day workers. The circadian 24-h average of activity in computer operators was significantly lower compared to that of the machine operators and day workers. Circadian amplitude of both machine operators and computer operators was significantly lower than that of day workers. Further, the peak appeared significantly later in machine operators (18.1h) than that of the computer operators (16.8h) and the day workers (10.9h). The value of r24 was also lower in permanent night workers compared with that of the day workers. Results of this study conclusively demonstrate that permanent night shift alters circadian rhythm characteristics of rest-activity in night workers significantly and the changes were independent of the factors age and to the most extent chronotype. These might force permanent night workers to face negative health consequences in future.