The relationship between objective assessment of sleep and momentary headache intensity in the daily lives of TTH patients was investigated prospectively using actigraphy and computerized EMA, making this the first study of sleep and TTH using this approach.
Total sleep time measured by actigraph was significantly and positively associated with headache intensity on the following day, suggesting that TTH patients experienced more intense headache when they slept longer on the previous night. In other words, oversleep might possibly be precipitating or aggravating factors of TTH and may support a result of a previous study in which 12.9% of TTH patients reported that oversleep was a precipitating factor of TTH [5
]. On the contrary to this result, previous cross-sectional studies based on questionnaires and interview also showed that many TTH patients reported that insufficient sleep was a precipitating or aggravating factor of TTH [5
]. This inconsistency might be because those previous studies using questionnaires and interviews actually do not investigate the real relationship between TTH and sleep itself, but asked patients' belief about their TTH, i.e. what patients thought about their TTH and its associations with sleep, which might be true to the facts or a wrong assumption unrelated to the facts. To test the hypothesis that both insufficient sleep and oversleeping are associated with intense headache, models including the quadratic term of total sleep time could be used. However, they are difficult to test using weeklong data because there was not a sufficient number of nights and the estimated headache intensity by the model with total sleep time as a categorical variable (Figure ) did not seem to support that model with the quadratic term. There is also the possibility that precipitating or aggravating factors are different among patients; however, the fact that the model with the effect of total sleep time as a fixed effect had better fit than the model with it as a random effect suggested that the effect of total sleep time on headache intensity did not significantly differ among patients.
There was no significant association between self-reported hours of sleep and momentary headache intensity on the following day, which was inconsistent with the result of total sleep time. There are some studies that compare objective sleep parameters (measured by polysomnography or actigraph) and self-reported sleep parameters in various populations such as healthy subjects [24
], patients with depression [25
], insominia [27
] and chronic low back pain [28
]. The difference between objective and subjective parameters was often pointed out and it has been suggested that both parameters should be applied. In this study, although objectively measured total sleep time and self-reported reported hours of sleep were significantly associated on a within-individual basis, self-reported hours of sleep were significantly shorter than objectively measured total sleep time. The inconsistent results of the self-reported hours of sleep could be interpreted by assuming that evaluation by actigraph is reliable and self-reports are unreliable because the subject can not accurately recall how many hours they have slept. Recently, it has been pointed out that when people are not able to recall their experiences, they make their self-report based on their belief, making up for lack of memories [29
]. Therefore, the result of the self-reported sleep might be a consequence of systematic bias reflecting the patients' belief that "insufficient sleep leads to intense headache", which were somehow inconsistent with objectively measured sleep. When their momentary headache is intense in the morning, they might have thought that sleep might have been short based on their belief and recorded their self-reported hours of sleep shorter than the real hours of sleep. This bias might have attenuated the actual relationship.
Self-reported sleep quality was also shown to be associated with momentary headache intensity. Sleep quality is a fairly subjective concept and no objective measure of sleep quality has been established yet [30
]. Therefore, it is not possible to know if any physiological process underlies the association of self-reported sleep quality with momentary headache intensity. There is a possibility that feeling that they slept well itself may have an effect which makes their headache better. It was also pointed out that ratings of sleep quality may reflect factors other than sleep, such as mood states [30
]. In this study, there is also the possibility that headache intensity may affect the ratings of sleep quality.
There are some limitations in the present study. Firstly, one week may not be long enough for investigating within-individual relationships between headache intensity and sleep. This might lessen the generalizability of the results of this study. Secondly, the effect of sleep on headache intensity on the following day was analyzed in the present study, whereas sleep over several preceding nights might affect headache intensity. Headache might worsen with insufficient sleep for several days. Longer observation times and application of variables processed in other ways may provide more information on the relationship between sleep and headache intensity in TTH. Thirdly, percent reduction of residual variance was small for both sleep measured by actigraph and self-reported sleep. This means that only a small part of the headache variation could be explained by sleep. In addition, even though the effects of sleep-related variables were significant, their coefficients are generally rather small. Therefore, it is not clear whether or not these statistically significant relationships are clinically significant.