With zero knowledge of your company and culture, I can only offer very broad strokes advice on this;
- Burnout is real and it will happen to people that work like that. Explain clearly that burning out means their hard work, and other peoples hard work goes to waste.
- Consider ways in which you might be inadvertently encouraging this behaviour. Are your devs constantly stressing about output or profitability? Do you often email or slack people at weird hours? Are people expected to be on-call? etc etc.
- Offer an extra day or two off every now and then and make sure everyone takes it. My company has at least a couple of 2 or 3 day breaks throughout the year in addition to our leave entitlements, and they are absolutely mandatory for everyone to take. This does wonders for morale, and I'm shocked this isn't more of a widespread thing.
- Monitor and moderate their assigned work if you have to. If engineer X has 20 unfinished tasks this sprint and they're all due tomorrow, that's a problem for everyone. Make sure they know to speak up early if they're over burdened.
- If you can, offer some sort of employee assistance program. People use work to escape from home and personal lives they don't like, having someone to talk to anonymously may help a bit.