Everything we do requires energy - be it thought, emotion, movement, or the instinctive functions of the body. Our every psychic act uses up energy. We relate to life through the five senses; impressions come into to the brain through them. Psychic energy registers what comes in through the senses, giving us the power of registration and focusing attention.
Take being on an extended holiday involving a lot of site-seeing, for instance. At a certain point although we are seeing everything, nothing seems to register. This is because we have used up all our psychic energy. Yogic texts say that our main source of energy is the food we eat. Food gives us the energies of magnetism, motor electricity, reflex electricity, and sex. But food cannot give psychic energy, which is only replenished in sleep. Hence, so much importance is given to optimal amount and good quality of sleep.
We have a certain quantity of this psychic energy when we wake up in the morning; we must learn to use this judiciously. This is stored in a large centre (or reservoir) which in turn is connected to two smaller centres. Suppose, we are reading a very interesting book, we can observe that after about forty-five minutes of reading, we feel a little tired and are not able to absorb the true meaning. This indicates that one of the smaller centres is empty. It takes a few minutes for us to connect to the other centre which is full. The best thing would be to walk a little.
Once we connect to another centre, we feel a rush of energy and can again read for another forty-five minutes. Perhaps, the reason why schools have study periods of forty-five minutes; it is the optimal period one can pay attention for. Once the other centre is empty too, we reconnect to the first centre, which by then has collected energy from the large reservoir. If we have not given it enough time to refill completely, we feel tired and debilitated sooner than later. Most of us are so overloaded with mental work that we overwork these energy centres, leading to exhaustion and mental breakdown.
Yoga prescribes practices which help to refill this psychic energy very fast, but that’s for another time.