Reply to the letter by Min et al.

To the Editor:

We would like to thank the authors for their comments and interest in our study with carefully classified people from the general population with chronic migraine and chronic tension-type headache (1,2).

We agree that many other factors than those presented in our paper may influence excessive daytime sleepiness (EDS), such as snoring or the presence of obstructive sleep apnoea (OSA). However, the study did not include questions or tests for the latter conditions. Also, the overall sample size limited the number of variables that could be analyzed as potential confounders in regression analyses. Furthermore, due to the limited space in a short report, we did not report all negative findings. However, applying the χ² test in non-adjusted analyses, no significant differences (data not shown) were found in those with and without EDS depending on socio-demographics, body mass index ≥30, smoking, alcohol, caffeine, other sleep disorders, anxiety/depression, comorbidity of other disorders or medication use for other conditions.

We also agree that it would be interesting to investigate not only the association between headache and obstructive sleep apnoea, but also the possible association(s) between other sleep disturbances and headache (3,4). This requires future longitudinal studies in the general population with the use of neurophysiological examinations such as polysomnography and/or the multiple sleep latency test, validated questionnaires, instruments and classifications (5,6).

Many previous studies have been based on highly selected patients, questionnaire-based diagnosis and self-made assessment instruments, making the studies difficult to compare and hypothesis-generating at best. In addition, the differences in the prevalence of headache diagnoses and EDS in different studies may partly be explained by differences in study designs, different cut-off for EDS (ESS ≥ 10), and/or use of different headache classifications.