joziel makes a good point!
"Yuppy flu" is what the medical profession in the late 1970s onwards decided fatigue as

. Usually sets in after a virus which depresses the immune system and causes intense exhaustion. I am told that gentle exercise when ever the sufferer feels able - too much and the body goes into free fall to build up energy again.
Classed as 'depression' it was treated as such - never getting to the causation. There had to be something more as people were affected World Wide. The body heals whilst sleeping which is why animals curl up for hours, however being in a society where appearing in the work place is expected, sufferers dragged themselves out of bed until the body said 'no more'.
My aunt was diagnosed with everything even to the point of having a hysterectomy. Clutching at straws her GP was, for over 20 years. She became withdrawn and unsociable.
Being hydrated: the pee should never be really pale as this can lead to other problems within the body. The 2 litres a day was pushed by the bottled water companies. We obtain liquids from most of what we eat/drink so keeping the urine the colour of straw is about 'right'. Any darker and we need to drink more.
Half a day at a time? Try to get into the sunshine when possible to top up VitD levels as low levels can cause weariness.