One gets drained because one is expending much more energy not commensurate with the receptivity. This happens commonly because the balance between work and rest is disturbed. The nerves get frayed due to an excess stimulation (information overload with WhatsApp and other things) and if sufficient time is not given to restore the body then it gets easily fatigued. Sufficient rest is restorative but it is not just about the number of hours we spend in sleep. It is the surroundings and the quality that is of greater importance.
Sleep is a period when our systems are being overhauled and the wastes removed. It is also a vulnerable time when one could be attacked by forces from the vital worlds, especially if one has gone to sleep after an animated discussion or intense excitement. The Mother advises us to enter a state of inner quietude before sleep. One could also imagine a cocoon of white Light surrounding oneself.
For resting too one can take the help of imagination such as floating on a river as a water Lilly. One can even imagine growing vast as the sea or the sky. Repeating one’s mantra before sleep and imagining white Light full of peace flowing into oneself is also very helpful.
One can draw energies during work itself by bringing in a certain degree of impersonality and universality by doing works as an offering and service to the Divine.
Finally there could be other reasons of a more personal nature especially the people we interact with and the places we visit during the day. Of course one needs to exclude certain physical conditions but for that one needs to see a physician and get checked up. Similarly if there is a strong undercurrent of depression that needs to be discussed separately with a counselor to sort out all the different issues involved.