Fatique, lethargy, headache, unexplained tiredness, extreme sleepiness amongst others, are common symptoms of the underrated and always overlooked "Dehydration"... or lacked of water (Please read: Dehydration) which is a common prognosis of 'burn-out' in most if not all new-athletes. We tend to always discount the importance of water.
There are three main types of dehydration: hypotonic (primarily a loss of electrolytes, sodium in particular), hypertonic (primarily a loss of water), and isotonic (equal loss of water and electrolytes).
A simple rule of thumb, to avoid that feeling of burning out after a prolong period of exercising and to prolong the energy for a consistent training without burning out, replenish the body with at least a can of isotonic drink (such as 100+, Gatorade) and twice the amount of isotonic drink with water immediately after exercising... of course with sufficient sleep too. Lack of sleep will lead to other problem, click Here for earlier blog-post regarding sleep.
dizziness when standing due to orthostatic hypotension, and in some cases, insomnia, groggy or sleepy, tingling in one's limbs (paresthesia), muscle spasm, constipation etc.... it is an unnoticed prognosis of Dehydration.
Those symptoms are due to the electron/Ion imbalances in the brain which can be avoided with the combination of sufficient isotonic drink and water.
Let us be consistent in our training without burning out by taking extra precaution with our water intake... Drink at least 4.5 liters (equivalent to 3x big bottle of coke) of water daily.
Nonetheless, it is imperative that we take all precautionary measure to avoid burning out and 'breaking' our training momentum.