Darkness affects the mood and stamina of all humans because our biology evolved alongside light, reports Inverse.

Winter months have fewer hours of sunshine and shorter days because of the way the Earth’s axis is tilted towards the sun. These extra hours of darkness confuse the body because they signal to the brain that it’s time to go to sleep earlier than usual.

When it’s dark, the body releases a hormone called melatonin from the brain’s pineal gland, signaling that it’s time to feel drowsy (for this reason, melatonin supplements are often sold as a sleep aid). Because light impedes the rate of melatonin secretion from the pineal gland, melatonin production increases at night. But during an especially dark winter, the body gets confused, producing melatonin as those 5:30 pm sunsets hit — much earlier than normal. That’s how you end up in sweatpants by seven.