The speaker discusses the critical role of light exposure in regulating our circadian rhythms and its impact on mental health. They highlight that getting enough sunlight early in the day, even on cloudy days, and maintaining darkness at night can help mitigate negative effects associated with mental health disorders such as depression, OCD, ADHD, and bipolar disorder. This approach helps in setting the body's physiological processes correctly, aligning with the natural 24-hour cycle dictated by light exposure through the eyes. The speaker also points out that the use of phones and screens can disrupt these natural rhythms, affecting mental health not only through the content consumed but also due to the light emitted by these devices. They stress that managing light exposure is a cost-free way to enhance mental health and overall well-being.