Our findings support the idea that the negative impact of winter on mental distress is –for the majority of the adult population- more a myth than a fact, or at least difficult to substantiate. 7.4% of the study population had a high level of mental distress. This is lower than the average for the general population in Norway (11.4%) . There were no significant differences in the reporting of current mental distress depending on season. Asking for current sleeping problems and current symptoms of mental distress lowers the risk of memory distortion and recall bias, which is one of the problems with retrospective studies [15, 22]. In our study, mental distress appeared to be a stable phenomenon independent of season and the extreme variations in light and darkness found in the sub-arctic. One could expect that season should be of importance to the level of mental distress, i.e. as in SAD. Moreover, one would expect a higher prevalence of sufferers of SAD at higher latitudes [14, 23, 24]. There seems to be less correlation between prevalence rates of SAD and latitude than expected [25, 26], and other factors like climate, genetic vulnerability and socio–cultural context may play a more important role . Saarijarvi et al.  found that the prevalence of overall and winter SAD did not differ between Finns living in northern and south-western Finland and that winter SAD was less frequent among the Sami. They suggest a genetic selection effect which could lead to better tolerance of the arctic environment.
The polar night in Tromsø is not completely dark, and the outdoor daylight intensity (usually increased by reflecting snow) on bright days at the beginning and the end of the dark period may be strong enough to minimize the seasonal effects on mood. However, we do not know whether our studied subgroup may be extra resistant to seasonal variations in light exposure.
The winter attendees in our study reported significantly more current sleeping problems than the attendees in other seasons. This corresponds to the finding of a different recent study involving people in the sub-arctic . It is possible that latitude is of importance to the prevalence of sleeping problems, as the same study did not find such problems in a sample of people living at 5 degrees North. Moreover, in a Norwegian study south of the Polar circle (at 63°-65° N), no evidence of a seasonal variation on reports of insomnia symptoms or time in bed was found .
Drawing on these findings, it is possible to hypothesise that seasonal variations in light-darkness are of greater importance to the prevalence of sleeping problems than to the prevalence of mental distress.
There are some important limitations in this study that should be mentioned. First, our study is limited to the adult population. We know from other studies that the level of mental distress is higher among adolescents, young adults and the elderly . Our study did not include persons below 30, and the quite low response rate for the youngest and oldest age groups could lead to an underestimation of the level of mental distress. However, we have no reason to believe that the age span in our study would mask any seasonal differences in mental distress.
Second, there may be a selection bias in that non-attendees may actually suffer from more mental distress and sleeping problems than those who attended the study. Hansen et al. found that non-attendees to a general health survey had more than twice the risk of attendees of having a psychiatric disorder . Knudsen et al. found that the risk of being a recipient of disability pension for mental disorders was three times higher among non-attendees . Sivertsen et al. found that insomnia was an independent risk factor for long term sick leave . If we transfer these findings to our study, there is a possibility that some people with high mental distress or severe sleeping problems during winter were too affected to participate in the study. However, none of these studies showed seasonal differences in participation rates related to mental distress or sleeping problems. Unfortunately, we had no data for attendance rates by month or season in our study.
A third limitation is that our data were collected at one single occasion, so the seasonal variations were based on the recordings at the different attending dates.
In a population study like the present, there is a possibility of reporting bias. A fourth limitation is that the attendees were not clearly blinded to the research questionnaire, as the statement of which time of year they suffered from sleeplessness could lead to a willingness to confirm the general myth. Although a higher proportion of individuals reported insomnia when queried in the winter, the differences did not appear to be as large as could be expected. This possible reporting bias could be a significant factor that might account for the different findings of the Nord-Trøndelag survey  and the Tromsø study.
One strength of the present study is the relatively high number of attendees. A high n makes it easier to detect any patterns in mental distress and sleeping problems. The overall response rate was 65.7%, which is relatively high for this type of study and which also ensures that the findings were representative of the population. However, the response rates for the young and the elderly were somewhat below the average, which could be a source of bias for these sub-groups.
Another strength of the Tromsø study in general is that it covered a variety of topics, and questions about mood and sleep were only a minor part of the large questionnaire. There was no focus on mental distress or sleep when information about the study was mentioned in the local newspapers. This could minimize the possibility of selection bias.