Maybe it is connected to the circulation or blood pressure going down and then getting up too quickly. Before I started practicing WILDs, I was always very groggy after taking a nap during the day. But when practicing WILDs I consciously do relax deeply while staying aware. Now I am usually refreshed after "meditative" naps, if I succeed having a WILD or not.
It might help to first start wiggling ones toes and fingers, then stretch and get up nice and slow.

It might be also connected to how you practice WILDs. As when I stay in bed in the morning for much longer, just lying there half asleep. I actually start feeling more tired and groggy when getting up, this can last for almost the whole day.