The energy from the sun can significantly affect our physical, mental, and emotional cycles, according to author Sherry DeWalt. The circadian rhythm regulates sleep and wake cycles in humans and can lead to difficulty sleeping at night. Seasonal affective disorder (SAD) occurs at the same time every year, generally in the winter, and sunlight tends to enhance its production of serotonin. Vitamin D is also crucial for bone health, and it takes 5-15 minutes to produce enough vitamin D.