Standing posture is not the issue. Neck stability is. When you sleep, your neck muscles do not support your neck vertebra and can cause muscle spasms and headaches it your head and neck are not straight.

The injury is what I call a subtle neck injury. It does not show on X-rays except when taken by a very specialized person and technique.

The treatment is to sleep with your neck straight is all directions.

But, if you were fine until yesterday and this morning, just be careful with your sleeping posture for a week or so. And, don't do dumb things your friends challenge you to do until you have slowly strengthened your body.