Human beings live in a 24 hour environment, in which light and dark follow a diurnal pattern. Our circadian clock, the suprachiasmatic nuclei (SCN) in the hypothalamus, are synchronized with the 24-hour solar day through a pathway from the retina and synchronize our internal biological rhythms.
Rhythmic variations in ambient lighting affect behaviors such as rest during sleep and activity during wakefulness, as well as its underlying biological processes.
Natural light vs. artificial light
To understand the effects of light on human physiology, it is important to understand light. Briefly, light is radiation in a specific range of the electromagnetic spectrum.
During the day, outdoor light intensities can reach illuminances of up to 100,000 lx in direct sunlight. On the other hand, light intensities in closed rooms are considerably lower and lhe standard office lighting is only 500 lx and often lower.
Daylight availability depends on geographic location and season. In the time frame of human evolution, it is a fairly recent development that light can be available throughout the day through artificial light.
Artificial light allows to illuminate interior and exterior spaces. It comes in many forms, for example incandescent, fluorescent or light emitting diode (LED) lighting.
What happens in the retina
In humans, the known effects of light on circadian rhythms and sleep are all, without exception, mediated by the retina. The retina is a thin layer of nerve tissue at the back of our eyes that contains specialized photoreceptors.
There are two main types of photoreceptors, rods and cones. Even so, they are not the only photoreceptors in the retina. A small fraction of secondary neurons in the retina that also send information to the brain and express a pigment called melanopsin.
For a long time it has been thought that rods and cones mediate what is normally consideredvision” (see color, movement, spatial details), and that the melanopsin average the “other” non-visual effects of light, that is, the suppression of melatonin, the circadian phase change. and alert. However, it seems that this is not the case.
Effects that light has on the circadian clock and mood
In addition to natural light, humans are also exposed today to a considerable amount of artificial light. This is particularly the case in the evening hours, i.e. when the circadian system is most sensitive to light-induced phase delays.
Therefore, artificial light can delay the time of the circadian clock and, therefore, sleep. In fact, it has been repeatedly suggested that light from LED screens interferes with sleep and the physiological processes involved (eg melatonin secretion).
For example, Chang et al., for example, showed that reading a book from an e-reader for four hours before bedtime increased sleep onset latency, reduced nocturnal sleepiness, melatonin secretion, and sleep alertness. next morning.
By objectively assessing sleep with electroencephalography, Münch et al. found that light exposurehowever faint, for two hours, three hours before usual bedtime led to shallower sleep and poorer rest.
Other studies have reported that ownership and smartphone use before bed may be associated with:
- More self-reported sleep problems
- Decreased sleep efficiencylonger sleep onset latency, and poor sleep quality
- sleep delayswhich also shortens the duration of sleep
Mood variations have been shown to be influenced by the interaction between the sleep phase and the duration of previous wakefulness.
Light can affect mood directly modulating the availability of neurotransmitters such as serotonin, which is involved in mood regulation.
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