Gowns typically have two ties. One at the neck, and one at the waist. Tie them both, and you should be sufficiently covered.
It is necessary for them to have you wear only a single fabric covering in case you start to die, and they have to tear the gown off of you to bring you back to life. Your nakedness is required for when they shove chest tubes into your side, heart injections, heart leads, CPR, and other mundane things like that which will bring you back to life.
Where I think there is room for improvement is for hospital staff to inform patients how to properly pt on a gown. On occassion, I see Hispanic women who do not speak my language wearing a gown that's open in the front, and their boobs are hanging out. During such instances, I am embarassed that my institution did not properly instruct the woman on how to wear the gown, and I do my best to help preserve her modesty.
Other than that, the single fabric gown is essential for hospital staff--such as myself--to do a good job for you. There's nothing worse than having to cut, tear, and struggle to get the pajamas and undergarments off of a dying person who is nothing more than dead weight. It delays the situation by a few crucial minutes that could kill you.
If you get cold, just ask nurses for blankets.
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