Hello,

Human body needs adequate amount of rest to keep it functioning well for  a longer period of time. It requires high maintenance.
When we exhaust it more than its capacity we are damaging it and studying for 18 hours out of 24 hrs is exhausting our body. But there are few students who claim that they study for 18 hrs, but it is not meant for everyone.

Everyone has different strategy and brain. Some people get thorough with the concepts even in just one go but there might be people who need more than a day to get through the topic. So the duration for how long you study does not matters, what matters is that you get well to the concept you read.

So it is not important to study for 18 hrs.

Hope it helps.

All the Best!!