It may be due to low blood sugar on arising. Sometimes I get flickering in my eyes, when I first wake up.

Some drugs really affect the muscles that focus the lens of the eye, and with age, this is slower to accommodate.

Also if you sleep with pressure on eye, that eye may have trouble focusing at first when the pressure is released.

Dry eye? Dry eye can affect focusing too. When the cornea dries out, there can be blurring. Oxygen gets to the cornea from the tears, where it dissolves and then goes into those tissues.

The fact that it resolves, it a good sign. Continued problems should be seen by an eye doctor. And next check up report this to him/her too.

Some autoimmune issues affect the retinal attachments. But that would continue throughout the day. We have a friend with this. It is called Uveitis. If you are concerned, seeing an eye specialist is a good idea.