Most people with CFS and FM find they have better and worse times of the day. It's likely you can get more done, without intensifying your symptoms, by changing when you do things, so that you use your best hours for the most important or most demanding tasks. 

One person in our program wrote about exercise, "If I walk in the evening, I can make it around two blocks, but three has me collapsing. Early in the day, I can do three or more. I have a window between 8 and 11 in the morning that is best for most activity, both mental and physical."
 

Another person was bothered by the effects of brain fog on her ability to read and retain information. Studying in the morning, she was able to read for only a half hour a day and had trouble remembering what she read. 

Then she decided to experiment with studying in the afternoon. She found that she had good mental stamina for several hours if she started the afternoon with a brief rest. After her rest, she could read for two 30-minute sessions with a short break in the middle and retain the information. 

Over time, she expanded her study time to a total of two hours a day. Experimenting with time of day enabled her to increase her study time greatly while also increasing her comprehension.