I remember very vividly this quote of Rilke about his sleep behavior mentioned in one of his letters (quoted from memory):

12 oder 18 Stunden - mag er nehmen was er will! ("er" refers to "Schlaf")

meaning that he does not over-regulate his sleep behavior and let the sleep take what it needs - let it flow...

Question:

In which letter does Rilke mention this quote and what does it say precisely as this is quoted from memory.

I am pretty sure it was contained in the volume of letters of Rilke I possess and which I photographed in this question - but it might be somewhere else as well, as a duplicate or original.

All references to this passage are welcome!