Yes your teeth can move overnight even though it may not be noticeable at first.
However after a few years you may be able to notice more of a gap between your teeth or notice your teeth becoming crooked.
To keep your teeth from shifting overnight you should have a retainer fitted by a dentist which keeps your teeth aligned and prevents the teeth from becoming crooked.
Most dental retainers last for up to 3 years before you need a new dental retainer.
Sometimes the retainer may need to be fitted properly again after awhile but never remove the retainer yourself until the dentist says you can otherwise the teeth can shift back which defeats the purpose of the retainer in the first place.
It can take some time to get used to wearing the retainer most of the time but after a few months of wearing the retainer you should be more used to it.
When you have a dental retainer put in by a dentist the dental retainer lasts from 1 to 3 years and sometimes you'll need to replace the dental retainer within a year but sometimes they can last up to 3 years.
Be sure to lookout for signs of wear and tear of your dental retainer and if there is wear and tear or the dental retainer no longer fits then you might need to get a new retainer that fits.
If you want to keep your teeth aligned you'll usually need to wear the dental retainer for the rest of your life and will need to replace the dental retainer within 1 to 3 years.
It's a good idea to see your dentist and they can tell you whether your retainer needs replacing or not and can easily replace your retainer for you.
But if you've had the same dental retainer for 3 years then it's likely time to get a new dental retainer because it's likely wore out.