Experimenting with myself, I have learned that if I sleep more than 8 hours in a row I wake up more anxious and depressed and tired.
On the other hand the few times I have slept only 4-5 hours I woke up rested and without anxiety but was more tired in the evening.
I then realized that if instead of sleeping 12 hours, I set the clock alarm to wake after 6 hours (then I would drink some water, read something and maybe have a snack) then I would fall back to sleep and sleep another 6 hours (setting the alarm again) I would feel better when finally waking.
I found out then that sleep deprivation is used as a cure for insomnia and depression. The problem seems to be a REM phase that is too long.
REM, I learned, is as tiring for the sleeping body as rope jumping is for the awake body and even more for us with CFS. REM depletes the body of energy and doesn't allow deep sleep. In fact during the REM phase we overdream, the brain is bombarded with sounds and images and real restorative sleeps can occurs only outside of the REM phase. Your orientation response gets exhausted so in the morning you can't change your focus of attention but keeps ruminating about scenario that couldn't be solved in the over-dreaming phase, and this rumination, I read, it's what depressive thinking is all about.
In other words we're susceptible to PEM caused by over-dreaming because of a long REM phase as we're to PEM because of over-training.
Sleeping many hours in a row, more than the body actually needs to feel rested - and we PWC are not exception because more sleep doesn't mean faster recovery or feeling more rested - increases the length of the REM phase and make us tired and more anxious and depressed. After all dreaming is considered "fake rest" it's actually more tiring than many wake life physical activities. If you think about it, you'll probably remember a pattern of "dreaming weird" or "dreaming a lot of stuff" when you feel more tired and more depressed.
Another reason to avoid sleeping too many hours in a row is the circulatory system (expecially for those with POTS, NMH and OI) Changing position, activating the blood flow in the arms by moving them, taking 30 minutes from the sleep position and then go back to sleeping, is probably another reason why taking rests from sleeping makes you feel better.
Another reason again could be low blood sugar due to running out of glycogen, which is abondantly used in the REM phase, which it could be a good idea to wake up and have a snack before going back to sleep and preventing an excessive sleep lenght.
I wrote how I need to rest every 5 minutes when I do whatever activity.
It makes sense that the same is true of sleeping (not 5 minutes but we need "rest" from it as well) since it is an activity, one that uses lot of calories, body energy and brain power. Just wanted to share this info with you.