This issue of Neuropsychobiology is devoted to chronobiology, that aspect of neuropsychobiology which concerns the study of circadian and seasonal rhythms, their modulation of brain and behaviour over time, and, in particular, the link to mood state. The rediscovery of seasonal affective disorder (SAD) in 1984 stimulated an enormous range of research protocols based on very specific hypotheses, ranging from photoperiodic causes (e.g., melatonin duration and photon count), phase delay or abnormal phase relationships between sleep and the circadian system, retinal subsensitivity (or supersensitivity), serotonergic mechanisms, and recently, genetic associations, particularly with clock genes. This new field was triggered by the key discovery that light could modify human circadian rhythms, and the fascination with SAD was linked to the possibility of a rapid and efficacious treatment. Light therapy is, in fact, the first treatment in psychiatry that has developed from basic neurobiological research. Thus, even though for many patients the attraction is the nonpharmacological nature of the treatment, we now know that light has very powerful and complex effects on the central nervous system, acting on multiple nonvisual systems to synchronise circadian rhythms, stabilise and improve sleep, and influence mood, alertness, and performance. The papers in this special issue cover a range of topics, from light treatment (Has physician use increased over the last decades? How can we improve outcome through patient behaviour? Can architecture - availability of daylight - influence depression recovery?) to food choices and SAD, how light affects metabolism, and how optimised bright-blue-enriched morning light can be used to counteract circadian desynchronisation.

The initial epoch of light therapy research has now expanded into the treatment of many other disturbed sleep patterns in psychiatry and medicine. It has enhanced the awareness of the importance of daylight (https://daylight.academy/), changed lighting regulations to include the nonvisual input, and is becoming a factor in “healthy” architecture and urban planning.

In 1988, the Society for Light Treatment and Biological Rhythms (SLTBR) was founded to bring together basic and clinical researchers in this common theme of seasonality and light therapy as well as to create standards and guidelines for treatment unbiased by commercial interests. The abstracts for the 29th Annual Meeting of the SLTBR in Berlin, which takes place around the midsummer solstice, 2017, are printed here, and they reflect the breadth and differentiation of the field today.

Anna Wirz-Justice, Guest Editor, Co-Founder and former President, SLTBR

Mirjam Münch, Associate Editor, President, SLTBR

2017
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