There could several reasons for this problem.
When a person does not eat enough food, he/she will become energyless. This alone can make the person lazy.
This perhaps is the brain’s way of conserving the last bit of energy the person may have, since the body would need some minimum amount of it for its vital functioning.
To overcome laziness, one may need to eat more of energy-giving food.
Taking a hot beverage, such as coffee for instance, would make available some amount of instant energy, and the laziness would disappear on its own.
Non-diabetics could take (only occasionally) a glucose drink to overcome this problem.
If a person happens to be in a room with little ventilation for long hours, he/she could become lazy.
This happens for want of oxygen, for it is oxygen that combusts (burns) the food to release energy. This can happen even if the person has eaten stomach full.
Some deep breathing exercise in a place with fresh air would replenish oxygen shortage almost immediately.
While sleeping, if a person happens to cover his face (i.e., nose) with a blanket or comforter, he/she would become much lazier when he/she wakes up. This again is due to reduced inhalation of oxygen, and increased uptake of carbon dioxide.
If a sedentary person habitually does shallow-breathing, he/she would not get enough of oxygen, and that would keep him/her lazy all the time.
A sports person, as long as he/she is active, would never feel lazy, for he/she would be breathing deeply all the time, which would ensure a very rich supply of oxygen for the release of the needed energy.
See Memory Failure, Stupidity.