Winter Colds Last Longer

Suppose a hamster is injected with bacteria which makes it sick - but in one case the hamster is on an artificial day/night cycle that suggests it’s summer; in the other case it’s on a cycle that suggests it’s winter. If the hamster is tricked into thinking it’s summer, it throws everything it has got against the infection and recovers completely. If it thinks it’s winter then it just mounts a holding operation, as if it’s waiting until it knows it’s safe to mount a full-scale response. The hamster “thinks” this or that?? No, of course it doesn’t think it consciously - the light cycle acts as a subconscious prime to the hamster’s health management system.

via, by way of gwern (recommended reading)