Give / force them some time off before they can start another block of 40 hour work.
"Flow" is mentioned so often, but I'd really like to see some good quality research to back it up. For sure people feel like they're in a zone, but how much of the code produced in that time is great quality, and how much is error-strewn garbage?
The need for standard weeks is still pretty strong, but that's not insurmountable problem. Really, when people talk of weeks they mean "continuous chunk of work with no 48 hour break".
A bigger problem is making people think that 60 hour weeks is normal, or honourable, and that a 40 hour week is lazy or disloyal or unproductive or harmful to the company. That's not true, and it's possibly leading to early death of some people. It's certainly making some people miserable.