This brought memories of my childhood... We kept the hot water bottle by our feet. The water bottles were put inside a fabric pouch (sort like a pillow case, but smaller and made of thick fabric). That helped to keep it warm well into the early morning, when it was time to get up for school... Our house didn't have any heating other than a fireplace in the living room which we put off before going to bed. I recall we had three layers of blankets but without the water bottle it never warmed up, don't know why... That was in the southern part of South America where winters are short (about 3-4 weeks below 10C)so it's not worth having a full heating system. If outside it was 5C, indoors would be 7C max... Now I live in Canada with good heating so all that is just a memory... Back then it would have been hard to believe I'd be wearing shorts and a T-Shirt indoors, in the middle of Canadian winter like I do now, God bless technology...

It was placed in the bed to warm the sheets so as not to get into an icy cold bed.
It stays hot long enough for you to feel comfortable, allowing sleep.
Instead of shivering with the cold.

I use it to warm up the bed and then put it under the bed.