That's where personnel management comes in. I've got an undergrad intern at the moment, who has only a modest amount of lab research experience. In a month and a half, I've taught him molecular biology from scratch (beginning with the fundamentals of PCR) and from what i taught him, I gave him a list of 48 mutations. He designed primers to make those mutations, does the molecular biology, checks the sequences, then does the biochemical experiment on the enzyme that's being mutated. He's finished about half of the mutants. We are able to get this done because the experiments were planned out to be paralellizable and scaleable, and if something is finishing up when it needs to be picked up at the end of the day (like a transformation recovery), I do it, because he comes in early and I stay in late. I also drop in on the weekends to start cultures- but usually only briefly -, to make the most efficient use of his and my time. He is in usually around 9:00-9:15 and I make sure he leaves at 5:00 and I really get angry if he's around past 5:15 except in exigent conditions.
Bottom line: Even in Biology, you can restrict your working hours if you're a team player if you have good management skills.
If you're overworking. Since science entails failure that you cannot engineer your way out of - you will wind up burning out, since the working hard followed by failure is exactly an optimal way of conditioning laziness.
