I work for Fitbit. I don't speak for Fitbit; this is my personal experience. If you get a Fitbit with heart rate tracking, which nearly all have now, it won't rely just on motion. For me it's worked great, showing an awake period at the beginning or end of my time in bed. More on how it works: https://help.fitbit.com/articles/en_US/Help_article/2163#Aut...

If you buy directly from Fitbit's website, the satisfaction guarantee is 45 days so you have plenty of time to try it out and see how well it works for you.