If I were to guess, the correlation to happiness (if any) has more to do with how the extra hour in the morning is used. It’s likely that people find themselves with an extra hour in the morning and do something more fulfilling than when it was at night. Whether it’s more personal time, a nicer breakfast, or even just taking more time to get ready with less anxiety of being late - all can be fulfilling in their own right. Whereas that same hour before bed all to often becomes mindless screen time.