I didn't wear my invisalign tray for 20 hours. Will my teeth start to move?

Not wearing your Invisalign for 20 hours does lead to some movement of your teeth, but once you place the aligner back in, your teeth can be expected to shift back into their proper place in another 10-20 hours.  So, every day that you go without your aligner, you are setting your treatment back about 2 days.  This is not a big deal every now and then, but it is can be a huge deal if you do this for several days. Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMDReal Self 100

As long as you have been good for most of your treatment one
day of leaving the tray off should not affect things too much. You just want to
make sure that you go back to wearing the
tray as soon as you can and keep it for at least few days prior to beginning
the new set.