Not sure if this has happend once or it happens ever morning.
A couple of things could be causing it. 1st, you could have a problem with the tendons/ligaments in your foot that resolves with standing and stretching it out. 2nd, remember you move quite a bit in your sleep
, & you can pull and strain things even then. Either way if it goes away with minutes of normal activity, i would be reassured.
Answer assisted by Mike Moore, Medical Student