A random sample, comprising of 1000 subjects aged over 17 years, with 1:1 sex ratio, was drawn from the national population register of Finland. These one thousand subjects were sent a letter inviting them to take part in a trial focusing on the effects of dawn simulation on sleep. The study procedure was briefly described in the invitation letter. No reference to sleeping problems and/or seasonal changes in mood was made in the recruitment letter. Subjects who were not shift workers and who had not been hospitalised nor taking medication during the previous six months were eligible for the study. The first 100 eligible respondents to the invitation letter were enrolled in the trial, after giving their written informed consent. This trial was part of a larger, binational study whose report is currently under preparation .
The study protocol is in concordance with the Helsinki declaration and its amendments, and it was approved by the local ethics committee.
The dawn simulators used were 'Light wake-up system' prototypes (Philips DAP), in which the light source was placed behind a diffuser. Research assistant provided the study subjects with the dawn simulator and visited each subject's bedroom to ensure the correct usage of the apparatus. The assistant checked 1) that the bedrooms could be darkened (<1 lux), 2) that the dawn simulator could be placed on a bedside table, or equivalent, on the subject's side of the bed, at eye level, 3) that the distance to subject's eyes (approximately 30 cm) and illuminance were same in all bedrooms. There were no bed partners in the study subjects. With the dawn simulator the luminance is weak (<1 lux) initially but intensifies following a linear ramp, reaching highest level in 30 minutes. After that, the lamp could be switched off. The trial was preceded by two days of adaptation during which subjects were instructed to use the dawn simulator and adjust the maximum level of illuminance. They were asked to set it at 200 lx for the first morning, but allowed to change it to either 100 or 300 lx for the second. Finally, they were asked to stick to the preferred level and use it daily throughout the trial. The dawn simulator was thus to function as their alarm clock on workdays, and they were also instructed to use it in the same way, and set for the same time, at weekends. The trial lasted eight weeks: two weeks of simulated dawn were followed by two weeks without, and this cycle was then repeated once. Half of the subjects followed the opposite design, starting without simulated dawn. Subjects were instructed to contact the research assistant if they had problems with the dawn simulator. The assistant collected the study questionnaires and dawn simulators at the end of each study period.
During the study period all subjects lived in the Helsinki region or South Finland province (60°–61°N). The study was performed in two periods (time of sunrise-sunset): 4 October (7:33am–6:43pm) to 29 November (8:53am–3:24pm), and 10 January (9:16am–3:40pm) to 7 March (7:02am–6:02pm).
The main outcome measure was quality of sleep, assessed daily using the 14-item Groningen Sleep Quality Scale (GSQS; see Appendix 1 for the items on the scale) . GSQS score ranges from 0 to 14, a higher score indicating lower subjective quality of sleep. The scale was originally constructed to study sleeping problems of depressed patients. In a validation study with 80 depressed inpatients, the mean (S.D.) score on the scale was 6.0 (4.2) and Cronbach's alpha for internal consistency 0.88 [Mulder-Hajonides van der Meulen & van den Hoofdakker, unpublished data]. The GSQS has previously been used in a study 27 SAD patients , and 70 shift-workers . In general, if sleep is unrestricted and undisturbed, subjects score 0 to 2 points. A higher score (6 to 7) indicates a disturbed sleep.
At baseline, the subjects filled in the Seasonal Pattern Assessment Questionnaire (SPAQ; ), and the Morningness-Eveningness Questionnaire (M-EQ; . The SPAQ measures seasonal changes in mood and behaviour and includes a 6-item scale yielding the Global Seasonality Score (GSS), ranging from 0 to 24 points. The SPAQ criteria for subsyndromal SAD used in this study were those described by Bartko & Kasper . The M-EQ has 19 items, which are scored from a minimum of 16 points to a maximum score of 86 points. The scale is divided into five types: definitely / moderately morning type, intermediate type, and definitely / moderately evening type. In this study, a tripartite division was used: evening type (M-EQ score 16 to 31), neither type (42 to 58), or morning type (59 to 86).
Before the study, subjects were asked about their expectations on dawn simulation, and how do they think it is going to make them feel. The question was rated on a seven-point scale from "really worse" to "really better".
After the study, the participants were asked to rate how they felt the intervention had affected their general wellbeing on a five-point scale from "worse" to "better". Any possible side effects were also asked after the study from those who completed the trial.
Data were screened and analysed with the S-Plus 2000 Professional for Windows Release 1 (Mathsoft, Inc.). To estimate the intervention effect on the GSQS, a linear mixed-effects model (LME) for longitudinal data was used . The absolute value of the GSQS was set as the dependent variable, and treatment effect, treatment × time interaction, and baseline characteristics as independent fixed effects. The LME accounts for intra-individual variation in a repeated measures design by setting the variation as a random effect and treating each individual as a cluster in modelling. To test whether the treatment effect lasted after each intervention, a carry-over indicator was included in the model.