Effects of Bright Light on Sleepiness, Sleep and Melatonin in Industrial Work

sponsored by the Former Swedish Working Life Fund
June 24, 2001

Arne Lowden, Torbjoern Aekerstedt and Roger Wibom

Light treatment have in laboratory setting been shown to rapidly adapt subjects to night work by changing the phase of the circadian rhythm. However, the effects still need to be evaluated in real life settings before we can make further recommendations for its use.

The object of this study was to assess effects of bright light treatment on sleepiness at work and on sleep quality.

20 experienced shift workers at Volvo (Skoevde, Sweden) took part in the study. At the end of the study 15 subjects remained for analysis. Each subject was scheduled to work 4 weeks x 5 nights (Mon-Fri). Work started at about 24.00 h and ended at 7.00 h. In a cross-over design, subjects were exposed to bright light during one month in the spring or autumn. The light group (L) were instructed to enter a bright light room on all breaks. Fluorescent tubes in the ceiling gave indirect white light with an illuminance of 350 candela/m2, generated by full-spectrum light tubes with temperatures of 5000 Kelvin. The non-light group (NL) went to a similar room without bright light. The workers filled out a sleep diary, alertness ratings (2 hourly) and wore actigraphs (Actiwatch) three days before the start of the first night shift, during three night weeks and during the week after night work. Melatonin was measured in saliva (2 hourly) on the day before night work, the day of the first night shift and on three other nights, ending with measures taken on the first and fourth recovery day after the night shift period.

Ratings from the length of breaks revealed that subjects obtained about 20-30 minutes of bright light on each night. The breaks were taken at midnight and at about 03.00h. Results from the ANOVA showed a significant interaction effect of light and time across the night shift (p=0.0429). Sleepiness was particularly reduced at 04.00h and at 06.00h. The interaction of light, night and time revealed that sleepiness was most strongly reduced on the first two nights of each week (p=0.0365).

Melatonin levels were suppressed during work in the light condition (L=14.3+-0.9pg/ml, NL=17.6+-0.9, p=0.0365).

Melatonin levels were suppressed during work in the light condition (L=14.3+-0.9pg/ml, p=0.0215). The suppression was strongest at 02.00h. A significant difference between nights were observed (p=0.0001), both groups seemed to be adopted to night work towards the last night of work but the adoption seemed to be more rapid with the influence of bright light.

Mean sleep length after work showed a significant effect of light (L=6.23+-0.1h, NL=5.89+-0.1, p=0.0187). This difference remained when nap data (sleep diary) were added to sleep data (actigraphy). Napping added about 0.3h (5%) of sleep each day.

The effects of bright light seemed to be positive in shiftwork. Light treatment reduced sleepiness and supressed melatonin at work, and sleep length after work was easier to maintain. But data has to be complemented with more extensive studies of exposure to light towards the end of the night shift, seasonal effects and the readaptation to day-oriented life.

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