The evidence is convergent. Multiple independent sources reach the same conclusion, the underlying mechanism is well-characterized, and even the field's most cautious voices treat it as worth doing.
if you want to shift your clock, typically you would do that by you can take a hot shower and then that will have a cooling effect after the hot shower. And if you were to get into a cold shower or an ice bath if you have access to one, afterward there's going to be a thermogenic effect of your body increasing temperature.
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If you do it after 8:00 PM, it's going to make your day longer, right? Because your body and your central clocks are used to temperature going up early in the day and throughout the day and peaking in the afternoon. If you then increase that further or you simply increase it over its baseline at 8:00 PM after temperature was already falling, even if it's just by a half a degree or a couple of degrees or you do that with exercise doesn't have to be with the ice bath, you are extending, you are shifting forward your phase, delaying your clock.
If you introduce an increase in body temperature by way of cold exposure early in the day, let's say 6:00 AM or 5:00 AM if you're masochistic enough to get into a cold shower at that time more power to you, it's going to make, you want to wake up about half hour to an hour earlier the next day than you normally would.
If you do the ice bath early in the day and then get out you will experience a more rapid rise or cold shower early in the day, a more rapid rise in your body temperature that will phase advance your clock and make it easier to get up early the following day.
So a cold shower first thing in the morning will wake you up, but that's waking you up in the short term because of a different mechanism which I'll talk about in a moment, but it also is shifting your clock, it's phase advancing your clock in a way that makes you more likely to get up earlier the next day, okay?
If you do it after 8:00 PM, it's going to make your day longer, right? Because your body and your central clocks are used to temperature going up early in the day and throughout the day and peaking in the afternoon. If you then increase that further or you simply increase it over its baseline at 8:00 PM after temperature was already falling, even if it's just by a half a degree or a couple of degrees or you do that with exercise doesn't have to be with the ice bath, you are extending, you are shifting forward your phase, delaying
If you do it after 8:00 PM, it's going to make your day longer because your body and your central clocks are used to temperature going up early in the day and throughout the day and peaking in the afternoon. If you then increase that further or you simply increase it over its baseline at 8:00 PM after temperature was already falling, even if it's just by a half a degree or a couple of degrees or you do that with exercise-- doesn't have to be with the ice bath-- you are extending, you are shifting forward, you are phase delaying your clock.
Maybe you start to find that using cold exposure early in the day is great for you. But using it late, if it's too late in the day, that's not great.
But it also is shifting your clock. It's phase advancing your clock in a way that makes you more likely to get up earlier the next day. It's going to make you want to wake up about half hour to an hour earlier the next day than you normally would.
So while some will recommend a warm bath or a shower soon before bed, a properly timed cold exposure may be even better.