As others have said, you likely have S.A.D. Seasonal Affective Disorder.

Basically, what remains of our hibernation instincts kick in. They want you to eat, stay warm and hidden, while conserving energy. Unfortunately, this is not conducive to modern life!

You basically need to trick your body into accepting its summer.

• Vitamin D helps a lot, though not always on its own.

• Bright, full spectrum light. Normally sold as SAD daylight therapy lamps. Do not skimp on the lumens!

• Heat. A radiant heater, matched with the Light does well. Together, they trick your brain into thinking it’s a good day to be out. Also try and avoid sustained cold, if you can. (Be VERY careful if you put a radiant heater on a timer. They can be a MAJOR fire risk, e.g. with a t-shirt too close from the night before.)

• Good sleep hygiene. Avoid blue light etc around bedtime. Also keep a stable wake time, even if you don’t technically need to that day.