Any ideas why my finger lock up overnight in a curl?

Trigger finger: Most likely cause would be trigger finger with locking of the flexor tendons at the a1 pulley. Occurs commonly at night due to sleeping in flexed position and the tendons locking up proximal to the a1 pulley requiring passive extension with the other hand upon wakening. Seek care with hand surgeon for possible steroid injection or surgical release.

Triggering: Sounds like you have trigger finger. Swelling on the tendon makes it too large to fit through it's pulley system. See a hand surgeon for evaluation. Anti-inflammatories, steroid injections and surgery are possible treatments.

Trigger finger: Generally, locking of the fingers or thumbs are caused by trigger fingers. This is a benign condition in which the tendon becomes entrapped around a structure called the A1 pulley. If this is painful, an injection or surgery can improve the symptoms. If this is not painful, you can live with this as long as you have full range of motion and full function.