You can ask, but be prepared to receive a no. It depends on your company and your boss though. The key thing is everything is open to negotiation. If you can present a solid argument to the company why working reduced hours would be a benefit to yourself and the company, you're more likely to receive it.
Example of things are talking about self care and looking after yourself. Would they prefer an employee who can work 100% for 30 hours or an employee who can only work 80% for 40 hours? Would they prefer someone who tries to solve their issue with small time off now, or wait and let your issues develop further so you have to take actual medical leave instead?
Be prepared to receive reduced pay, less benefits (E.g. holiday allowance) and other things. Also, if you are in a employed in an at will state, an employer could take it as a sign of weakness and decide to let you go to find an employee in better health. However, you may be protected by law and regulations, but you'd need to check your local legislations.
I've known several people who have gone down to reduced hours, due to home commitments or self care, and have found it's allowed a new lease on life. This is in Western Europe though and not all shoes fit all feet!