This study revealed an inverse association between sleep duration and the risk of breast cancer in Japanese women, participants who slept 6
h or daily having a significantly increased risk of breast cancer.
There have been three prospective cohort studies of breast cancer in relation to sleep duration (Verkasalo et al, 2005
; Pinheiro et al, 2006
; Wu et al, 2008
), of which the last two reported a significantly decreased risk in long sleepers and our results are consistent with these. By contrast, another study reported no such association (Pinheiro et al, 2006
), possibly studied because residential nurses were studied with rotating-shift work and varying timing of sleep, so that generalising from their results may be inappropriate.
Melatonin is suggested to be involved in this relationship with sleep duration, a decrease that results in a shorter duration of nocturnal melatonin secretion (Wehr, 1991
). A lower melatonin level was associated with an increased risk of breast cancer (Schernhammer and Hankinson, 2005
; Schernhammer et al, 2008
). Melatonin may have an inhibitory effect on gonadal function, including the synthesis and secretion of sex hormones, by promoting the release of gonadaotropin-releasing hormone (Martin and Klein, 1976
; Aleandri et al, 1996
); it also exerts an antiproliferative effect on breast cancer cell lines (Blask et al, 1997
Our study had several strengths. First, we recruited participants from the general population, allowing possible generalisation of our results. Second, the Miyagi Prefectural Cancer Registry is one of the earliest and most accurate population-based cancer registries in Japan (Takano and Okuno, 1997
), with only 2.7% of breast cancer cases ascertained by death certificate only (DCO) in 1998–2002 (Curado et al, 2007
Our study also had several methodological limitations. First, we used self-reported sleep duration, and the assessment was done only once. Second, we had no information on such factors as sleep quality, the timing of sleep, the use of sleep medication, or the presence of sleeping disorders that can influence sleep duration and thereby might affect breast cancer risk. Finally, we had no information about rotating-shift work or night work, but since 23% of our participants were housewives, 19.0% farmers, and 15.7% retired, such details would have been unlikely to have changed the result substantially.
In conclusion, we have found a significant inverse association between sleep duration and breast cancer risk in Japanese women, those who slept 6
h or less having a significantly increased risk.