I've been a solopreneur and founder of a bootstrapped startup for 8 years. I reached 20k MRR this summer. The sentence "It's a marathon not a sprint" suits well my schedule and mentality. For the first 6 years of my bootstrapped startup I used to work 6-12 hours each day. (when I became dad 2 years ago it became harder to work few hours on the weekend though).

I used to work every day, also during the weekend. That's because I'm most efficient just after waking up, both in the morning and in the afternoon after a 30 minutes power nap. You might not be efficient in the morning but in general I recommend everyone to find out when it's the time of day when they are most efficient and work that time every day. Better to work less hours but working every day during the golden hours.

It's important that you love to do your daily work. In my case is coding, I love coding. If you don't love the core work necessary for your startup to succeed you will not be able to keep at it for several years.

I recommend always sleeping those 7-9 hours per night that make you feel full rested. There is no point in overworking now if you will be tired or burnout later.

If you are single and able to move, I recommend becoming a digital nomad and living in cheap/nice places. You can move to Thailand or Colombia or Portugal and pay less for rent and food and fun and all the savings will allow you a longer time to work on your startup. That said, even if you have more time you shouldn't waste it. Try to earn money from your project as soon as possible. In general it will take you 3-10 more times than expected to earn your first dollar. So if you have savings for 1 year you should do plan to earn your first dollar in 1-3 months.

The podcast Startup for the Rest of Us has been invaluable in my journey: https://www.startupsfortherestofus.com/