Read Rick Lane books on evolution and life...its leaky mitochondria. That is the answer. His book is an amazing amalgamation of 5 disciplines. And all based on real science with references.
that statement is factually not correct but I do think they are talking not of erythropoesis , which is 100-120 days...they are referring to recruitment from the marrow of mature RBC's which is about 4 days. The cells develop in the bone marrow and circulate for about 100–120 days in the body...
Read the book "Lights Out" by TS Wiley and Bent Formby. It will answer this and many more questions about how melatonin works in sleep. Melatonin used to be to the master switch for humans but now Leptin is because of ambient light. The more you know the better informed you will be to improve...