That's really not that surprising. If I feel that nagging 'it would be unhealthy to stay up any later' feeling, staying up and putting in those extra hours just seems less appealing if the alternative is a bed with the warm body of someone who loves you. Guess it's a matter of motivations/priorities.

Alternatively, maybe it just means that, once you have a family, you're less likely to take the risks that are sometimes necessary to produce those genius contributions.