Serotonin is what makes us energized and happy while melatonin is the hormone for sleep. When using Light Therapy, your eyes absorb the light which causes your brain to release serotonin. This is what helps give you energy and boost your mood – which is why you want to use your light in the morning. After only using your light for 20-40 minutes, your brain should produce enough serotonin to last you throughout the whole day, otherwise you may be suffering from a circadian rhythm disorder. When the sun starts to set and it is getting closer to bedtime, your body will subconsciously know to stop the production of serotonin and start making melatonin sleep hormones. When your brain produces melatonin sleeping hormones, you become more relaxed and sleepy. This cycle of your body knowing when to be awake and when to sleep is known as your circadian rhythm.