Light plays a central role in regulating the body's internal clock, known as the circadian rhythm, which governs sleep, mood, and biological cycles. Natural daylight synchronizes this system via the suprachiasmatic nucleus (SCN) in the brain. However, with the widespread use of artificial lighting and screen-based devices, circadian misalignment has become a common concern, contributing to sleep disturbances and mood disorders (Blume et al., 2019).

This disruption is especially significant when artificial light is encountered during the evening, a period when the body is most sensitive to light-induced delays in the circadian phase.

Systematic reviews and experimental studies have shown that short-wavelength ("blue") light suppresses melatonin, a hormone crucial for sleep initiation, and can delay sleep onset, reduce sleep efficiency, and shift circadian timing (Tähkämö et al., 2019; Chang et al., 2014). Even low levels of light (as little as 5–10 lux) during sleep can affect circadian rhythms. In real-world settings, bedroom light exposure above 3 lux has been linked to lower sleep efficiency and increased night-time movement, emphasizing the impact of ambient lighting on rest quality (Xu et al., 2024).

Despite these challenges, light is also a powerful tool for improving sleep and mood when used therapeutically. Bright light therapy (BLT) has been validated for conditions like seasonal affective disorder (SAD), insomnia, and other psychiatric or neurodegenerative disorders.

Studies show that morning light exposure improves sleep timing, enhances mood, and promotes circadian alignment, especially when delivered consistently (Blume et al., 2019). Innovations such as dawn simulators, therapeutic light boxes, and even architectural lighting design are now being explored as non-invasive strategies to promote sleep hygiene and emotional well-being.