Summarize this article:

You won’t be comfortable if you have to sleep with your body contorted into a pretzel, so lay down in the bathtub and see how well you fit.  In order to be comfortable for an entire night of sleep, the bathtub must be long enough for you to stretch out in and wide enough that your shoulders won’t be cramped and you can keep your spine in alignment so your back won’t hurt in the morning.  If the tub isn’t big enough, the floor may be a better option after all. Sleeping on the floor actually has some health benefits and can be good for an achy back!   Make sure you have room to sleep on your back but can also roll over onto your side so you won’t be uncomfortable staying in one position all night. Since people will also be using the bathtub for showers, you will need to make sure it is thoroughly dry before making your bed inside it.  Avoid using the bathtub for several hours before bedtime if possible. If the tub is still wet from someone’s shower, dry it with a towel. You can also use a hairdryer to completely dry the tub. Clean the tub so you won’t get any soap residue or hair on your blankets and pillows. You don’t want to knock a bottle of shampoo onto the floor or have a bar of soap fall on your face during the night.  Remove toiletries (shampoo, conditioner, soap, body wash, lotion, etc.) that are in the way or that you might accidentally knock onto the floor while you sleep. Be courteous with other people’s belongings and make sure you replace everything in the morning.

Summary:
Measure the tub. Make it dry and clean. Clear away clutter.