I’ve no idea in all honesty. It’s a recommendation I read elsewhere and it worked for me. I’m not sure of the ‘science’ behind the concept but I found it very beneficial. VR and a warm room are not an ideal match for multiple reasons.
You can’t buy an Index so there’s no point in torturing yourself. I see the Rift S is out of stock now in various places as well. With the situation in China it’s hard to see an improvement in VR headset stock levels anytime soon.