Have you ever experienced that when you are free, you have less time and when you are busy, you have more time? This may seem illogical on paper but practically it is all together a different story. Whenever I do more work than I am supposed to do, I feel active and energetic. Brain is unlike the machinery in a factory; the more you use brain the better it gets. There is no depreciation, or amortization. Imagine if money would work like that, the more you spend money the more you have. What is money actually? A piece of paper to let you live the life you want or to give you the knowledge you want or to make you a better person? If you think more deeply, you already have an asset which is better than money, the brain.
The brain grows in the same way the muscle grows. Muscles need a shock to tear the tissues so that they can build itself better, bigger, and stronger than before. We hit the gym to give the shock to our body but fears giving shock to our brains. While it may seem that you are weak today, your brain is exhausted, you are overloaded with tasks and you can no longer persist, but it will help you to grow. When you are thinking about whether to do this or not, somebody has already done it.
So try this, pick up a task which is brain wrecking. Do it. Do it for the 20 hours which need not to be in continuation but when you do that work, put all your concentration in. You will no longer feel stressful after 20 hours and the reason being that your brain will develop a new level of tolerance reaching its next level. The next level of success. The next level of enlightenment.