Blistering of a paint film is caused by either moisture in the wall or a chemical reaction with something directly under the paint film.

First look at the area(s) that have blistered. Go ahead and scrape a larger blister and see if there is any accumulated moisture. You also mention the morning sun. The morning sun shouldn't have any affect on the paint but will warm up the walls. Have you noticed the blistering on a sun facing wall? Are the blisters occurring on a sun facing exterior wall? This could be an indicator of moisture in the wall and it is being heated up and turning into a vapor under the paint film.

If not then it could be something on the wall when painted. Some things that can cause this are soap, air fresheners (both aerosol and plugin) , candles and greasy compounds.