Which psychological principle best explains why studying an hour per day for a week is more effective than one 7-hour study session?

Distributed practice

Explanation:

The distributed practice also called a spaced practice, in which a person breaks his / her time for learning in a broken session for a long period. German Psychologists Ebbinghous first observed distributed learning. Ebbinghaus studied a list of non-syllable to confirm the prior knowledge by allowing spacing effects and serial position effects.

For example:

When there is done an experiment in which a postman has given training of typing for a longer period then he learned in a short period called mass learning but when learning type through a space learning, it takes time but more effective learning than massed learning. spaced learning also called distributive learning.