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.

To address the problem effectively, consider these key concepts:

1. **Moisture and Heat**: Excessive heat from direct sunlight can cause the paint to dry too quickly, trapping moisture underneath and leading to blistering. Ensure the surface is not too hot when applying paint and consider using a paint with better heat resistance.

2. **Surface Preparation**: Proper surface preparation is essential to prevent blistering. Make sure the wall is clean, dry, and free of any contaminants before painting. Use a primer suitable for the wall surface to improve paint adhesion.

3. **Paint Selection**: Choose a high-quality, moisture-resistant paint that is suitable for high-heat areas. Consider using paints specifically designed for sunny rooms to minimize the risk of blistering.

4. **Application Technique**: Ensure the paint is applied evenly and at the recommended thickness. Avoid overloading the roller or brush, as this can lead to uneven drying and subsequent blistering.

5. **Professional Consultation**: If the issue persists despite proper precautions, consider consulting a professional painter or paint supplier. They can provide insights tailored to your specific situation and recommend the best course of action.