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Anatomical and neurochemical bases accounting for the frequent association between headaches and sleep: the hypnic headache paradigm
INTRODUCTION. The relationship between headache and sleep is complex and runs in two directions. Headache may be the consequence of a (primary or secondary) sleep disorder or its cause (in chronic tension-type headache and/or chronic migraine with or without painkiller abuse). It can also be related to sleep in an intrinsic way, as in the case of hypnic headache (which only appears during sleep) or other primary headaches, such as migraine and certain trigeminal-autonomic cephalgias (which can appear during the waking state or during sleep); this type of headache occurs mostly during REM sleep. DEVELOPMENT. The neural pathways that control sleep and pain are anatomically, physiologically and neurochemically cross-linked. These neural systems are located in the brain stem, the hypothalamus and the basal brain. Such cross-links are produced on two different levels. On the one hand, they occur in the serotoninergic nuclei of the brain stem, whose activity in physiologically diminished during REM sleep and which are involved in antinociceptive control. On the other hand, they are also to be found in the hypothalamus, where serotoninergic terminals have been detected in the suprachiasmatic nucleus (SCN). As cells in the SCN are lost with age, their circadian and homeostatic functioning fails, the activity of the hypothalamus-pineal axis is reduced and the endogenous melatonin rhythm is altered. This results in a disorder affecting the control of the sleep-waking cycle and antinociceptive control.