Yes for the best results you should wear the Invisalign tooth aligners while you sleep.
The more you wear the Invisalign tooth aligner the faster and better your teeth will straighten out so wear the Invisalign tooth aligner when you sleep is recommended.
The Invisalign tooth aligners are almost invisible because the Invisalign is made out of clear plastic.
However some parts of the Invisalign tooth aligners are visible but it can be hard to see that you're wearing the Invisalign tooth aligner unless you're right close to the person wearing them.
When using Invisalign to straighten your teeth it can take between 12 to 18 months to straighten your teeth with Invisalign.
You must wear the Invisalign as often as your dentist says too otherwise your teeth won't straighten or take longer to straighten.
Within 6 to 10 months you should see straighter teeth and after 12 months to 18 months you should have straight or at least pretty straight teeth.
For some people it might take a 2 years to see really straight teeth depending on how crooked your teeth are.
Just make sure you use the Invisalign and wear it as often as you need too so that your teeth properly straighten.