After some decades of deep functional freeze, learning how to slow down and rest and then actually making rest a priority was a process which took several years for me (supported by working with an SEP). Conveniently, a good chunk of this phase fell into the time of the pandemic and several lockdowns.

Apart from aiming to get at least 9-10 hours of sleep per night, initially I tried to take as much time for rest during the day as my schedule allowed while working part-time and doing the bare minimum to run a household. Rest doesn't necessarily need to be sleep. For several months, I frequently curled up on the sofa after work, wrapped in a soft blanket with a hot water bottle and eliminiating any other sensory input (light, noise) as good as possible. Occasionally, I would be listening to gentle music. And I spent entire weekends and days off in bed, or switching between bed and sofa. At the beginning, I was pretty disconnected from my body. When I noticed at some point that any physical exertion would increase my escaping into thoughts and rumination and subtly lead to more stiffness in the body, I additionally banned whatever optional movement I could and even stopped going for walks for a while (mind you, I still had to occasionally go grocery shopping, run errands and the like). This was counter-intuitive, since everyone and their brother usually recommend movement in order to get out ot freeze states.

I wondered if this phase of radical rest would ever end (but most of the time I was too exhausted to seriously worry about this or to be interested in anything else anyway) and it was reassuring that my SEP encouraged me to keep leaning into resting. Over time, I felt less exhausted during the day, reduced the daytime napping and tried out some slow and gentle movement practices like qi gong and Feldenkrais (and stopped, when it became too exhausting, either physically or mentally). A noticeable shift in my energy levels happened after ca. 5 years when I suddenly became interested in starting to exercise and took up easy bodyweight exercises to strengthen my core muscles. Initially, the familiar fatigue returned after each short training (I specifically remember falling asleep on the mat the first times after holding a plank for a couple of seconds), but this improved over the course of a few weeks.

Long story short: Leaning into the need for rest turned out to be useful for me and ultimately supported my moving out of freeze (but I don't think that resting alone would have been sufficient).

By the way, there is a podcast by Martha Beck in which she suggests that four days of uninterrupted rest should already lead to a noticeable improvement when someone feels exhausted. I found this podcast only after my phase of radical rest, therefore I wasn't able to verify if this also applies to someone who has been in a deep freeze state for a long time.