This chapter focuses on the biological effects of light on human physiology and behavior with an emphasis on brain arousal. Light exposure may be beneficial in situations where humans are physiologically vulnerable to the effects of sleep loss and circadian misalignment. The influence of light on human physiology and behavior has implications for public health and safety, especially for individuals who are required to maintain alert wakefulness during the biological night-time and for individuals who are required to maintain alertness and performance during periods of sustained wakefulness or restricted sleep. Light provides input for visual image formation and concurrently provides environmental information to modulate the physiology and behaviors collectively referred to as non-image forming (NIF) responses. The chapter reviews the alerting effects of night-time light exposure which are primarily observed during light exposure. Light has also been combined with other wakefulness-promoting countermeasures to enhance performance at night during sleep deprivation.