Exposure to sunlight naturally improves mood, energy, and alertness by affecting the body's physiology through hormones such as melatonin and serotonin. During the shorter days of autumn and winter, when people tend to spend more time indoors, a lack of sufficient light exposure can lead to feelings of tiredness, slowness, and irritability. Bright light therapy, which simulates natural daylight, boosts alertness, mood, and energy, and is an effective way to combat winter blues and Seasonal Affective Disorder (SAD). In these studies "the dose of light that has proved to be the most beneficial is **5,000 lux hours per day**, which could take the form of, for example, 10,000 lux for half an hour each morning." [_J Psychiatry Neurosci._](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC543845/#:~:text=J%20Psychiatry%20Neurosci.) #### The problem with most SAD lights is that they are so much less bright than daylight. This means you need to sit either very close to them, typically 5-30 cm, or look at them for a long time to reach 5,000 lux hours. Take, for example, the elegant [Lumie Task Light](https://www.lumie.com/products/task), which advertises 10,000 lux at 15 cm but only 1,500 lux at a more practical 50 cm. At this distance, a user would need to remain looking directly at it for 3 hours and 20 minutes to receive the required lux. ## Quality of Light - CRI The most common way to measure the quality of a light source is the [Color Rendering Index (CRI).](https://en.wikipedia.org/wiki/Color_rendering_index) This index quantifies how accurately a light source illuminates colors in comparison to a natural light source (like sunlight) on a scale of 0-100, with 100 being natural daylight. Typically, [a CRI of 80 and above is considered good, and over 90 is excellent.](https://www.takethreelighting.com/understanding-cri.html) Sunday achieves a 93+ CRI. In practice, this means that colors look brighter, people appear more vibrant, and it feels just like a sunny day. #### One of the key roles of light is to regulate our circadian rhythm (sleep-wake cycles) through melatonin suppression. The physiology of the eye means that certain spectral distributions of light have a more profound effect on regulating this than others. A measurement used to determine the effect light has on our biological clock is [Circadian Stimulus (CS)](https://docs.light-health.org/cscalc/about/background), measured on a scale of 0-0.7. ![[Pasted image 20240329170715.png]]