Over the last few weeks, I’ve started going to bed later, mostly because of some social events.

I shall return to my previous sleeping routine untillbefore it's too late.

Over the last few weeks, I've started going to bed later, mostly because of some social events.

I shall return to my previous sleeping routine untillbefore it's too late.

Over the last few weeks, I’ve started going to bed later, mostly because of some social events.

I shallould return to my previous sleeping routine untillbefore it's too late.

“Shall” also works if you are declaring it’s something you will definitely do. It’s a very proper word, which I enjoy. “Should” works well with “before it’s too late” because it implies there is another pathway that leads to less desirable results.

I shall return to my previous sleeping routine untillbefore it's too late.

Over the last few weeks, I started've been going to bed later, mostly because of some social events.

I would say I am a morning person, so I feel most productive in the morning, and it's better for me to wake up as early as possible.

I shall return to my previous sleeping routine untillbefore it's too late.

Shall sounds quite formal for this context. I don't remember the last time I've used that word.

I assume you meant to say "before it's too late"... You need to return to your old routine before it becomes too difficult to do. Staying up late and waking up early is not sustainable.