Bright light is an important influence on our body. Too little light in the morning may cause us to feel tired during the day. Too much light at night may make us unable to fall asleep at bedtime.
When our internal body clock does not match our external environment or lifestyle, we do not feel good. This is why we feel sluggish when we travel across time zones and experience jet lag, or during long dark winter days.
Our modern way of life, where we site inside our dimly lit office during the day and look at our screens on brightly lit devices at night, is disrupting our bodies' natural clock. This causes us to feel tired and moody.