As winter nights draw in I thought I'd leave food and nutrition this week and talk about the importance of daylight for our health and well-being.
The qualities of daylight change through the day with blue wavelengths being predominant in the morning and red in the evening. Our bodies and circadian rhythms have adapted to utilise this to our benefit, through two particular main mechanisms:
1) retinal ganglion cells (cells at the back of eye) that are not involved with vision, but take light messages to other parts of the brain.
2) the synthesis of vitamin D.
Morning light:
Vitamin D synthesis requires UVB light on our skin - and therefore is not made when we sit by a window as glass effectively blocks this wavelength. UV light is just beyond the blue end of the visible light spectrum, and there are greater quantities of UVB getting through the atmosphere when the visible light is bluer .. in the morning. Blue light wavelengths coming through the eyes to the retinal ganglion cells also trigger the production of Adrenocorticol Hormone (ACTH) in the pituitary gland and the suppression of melatonin (sleep hormone) setting in motion the day-time levels of our Hypothalamic-Pituitary-Adrenal (HPA) axis which keep us awake, stimilated, focused and with appropriate responses to acute stressors.
Evening light:
This is a softer, pinker light with significantly less of the blue wavelengths. It therefore is less good for creation of vitamin D, but also causes our retinal ganglion cells to send messages to shut down the daytime hormones and allow an increase in melatonin. It acts as a trigger to our body to prepare for rest, and is part of the process that decreases our heart reate, body temperature, energy levels, focus etc to prepare us for sleep.
Practical implications of this scientific understanding I think becomes fairly obvious once the basics are explained. We need to get good quality daylight particularly in the morning to maximise our vitamin D synthesis and to reset our circadian rhythm to the new day and stimulate all the "waking" hormones. Later on in the day and particularly in the evening we need to minimise our exposure to blue light in order to allow our body to fully rest and restore itself with good quality sleep. This means that we ought to think about timing our walks and outdoor time for first thing in the morning (for vitamin D) and then maybe again at dusk to help set us up for sleep - however we then also need to think carefully about screen and LED light exposure after this time to prevent the body reacting again to the blue light as though it is morning.
For further information these are some good articles I found whilst writing.
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