If your ortho has recommended this, then I recommend you do what you've been told. If you're feel you can't trust your ortho, you may be best to consult with someone else and change orthos ... of course this will involve cost, but you have to decide what you'd prefer to do.
Many orthos use 'nightime' only for some types of movement, so wearing elastics for this amount of time is not unusual.
I've had elastics for the past 3 1/2 years and it's always been full time wear ... miniumum of 23 hours.
Good luck ... but please don't do anything your ortho doesn't tell you ... it's not recommended and could put your results at risk.