The human body is amazing. If you keep gradually adding more and more stress, the body will adapt and get stronger and stronger. BUT ONLY IF YOUR BODY IS IN AN ADAPTIVE STATE. If you are not in an adaptive state, you might be able to train, but you won't ABSORB the training well and you won’t get faster.
You are likely NOT in an adaptive state if you are: * burning the candle at both ends * sleep deprived * on an irregular sleep schedule * dehydrated * stressed out * having a negative attitude * under fueled (not enough calories) * undernourished (not enough nutrients)