Daylight exposure and mood in real life: Direct association and mediating role of sleep and routine regularity.

The light/dark cycle is the main external cue to synchronize the human biological clock. Modern lifestyles typically lead to less daylight exposure and blunted 24 h-amplitude. We evaluated the association of outdoor daylight exposure (frequency, duration, regularity and shift) with chronotype estimated by sleep phase, regularity of routines, sleep, well-being (WHO-5), and depressive symptoms (PHQ-9), in a sample of 1,095 participants (81.8% female; 87.9% aged 18-49) surveyed online between July and November 2020. We analyzed direct and indirect associations in daylight-mood relationship with chronotype-estimate, routine regularity, and sleep as mediators. Outdoor daylight exposure was associated with WHO-5/PHQ-9 scores in mediation models, with higher total effects when the exposure was every day (ß = 4.13 ± 0.53/ ß = -3.81 ± 0.67), for more than 4 hours (ß = 3.77 ± 0.91/ ß = -3.83 ± 1.31) and during the morning (ß = 3.41 ± 0.53/ ß = -3.74 ± 0.70) in reference to lack of exposure. Chronotype-estimate, routine regularity score, and sleep problems acted as mediators, while social jetlag and sleep duration did not play an important role in this association. This study advanced the understanding of the complex interplay between light exposure, mental health, and individual characteristics of sleep and other routine regularities, and showed the benefits of optimizing daylight exposure to improve mental health.

Melatonin multiple effects on brown adipose tissue molecular machinery.

Brown adipose tissue (BAT) influences energy balance through nonshivering thermogenesis, and its metabolism daily and seasonal variations are regulated by melatonin through partially known mechanisms. We evaluated the role of melatonin in BAT molecular machinery of male Control, pinealectomized (PINX), and melatonin-treated pinealectomized (PINX/Mel) adult rats. BAT was collected either every 3 hours over 24 hours or after cold or high-fat diet (HFD) acute exposure. HFD PINX animals presented decreased Dio2 expression, while HFD PINX/Mel animals showed increased Dio2, Ucp1, and Cidea expression. Cold-exposed PINX rats showed decreased Dio2 and Lhs expression, and melatonin treatment augmented Adrß3, Dio2, Ucp1, and Cidea expression. Daily profiles analyses showed altered Dio2, Lhs, Ucp1, Pgc1α, and Cidea gene and UCP1 protein expression in PINX animals, leading to altered rhythmicity under sub-thermoneutral conditions, which was partially restored by melatonin treatment. The same was observed for mitochondrial complexes I, II, and IV protein expression and enzyme activity. Melatonin absence seems to impair BAT responses to metabolic challenges, and melatonin replacement reverses this effect, with additional increase in the expression of crucial genes, suggesting that melatonin plays an important role in several key points of the thermogenic activation pathway, influencing both the rhythmic profile of the tissue and its ability to respond to metabolic challenges, which is crucial for the organism homeostasis.