Even as many of us are trapped indoors due to the pandemic, our moods rise on bright days and plumb the depths after days of rain. Such transitory mood swings are not well understood but animal behavior offers useful insights.
Low moods are demotivating. We feel lethargic, or tired, and are less likely to exert ourselves in new, or productive, activities. While we may feel listless, dispirited, and unhappy, this could boost the biological currencies of survival and reproduction.
Knowledge about the restorative value of sleep is expanding by leaps and bounds, from its effects on immune function to memory and flushing the brain of impurities. Feeling tired encourages us to lie down and sleep each night.