there's a reason behind every action
Proverb meaning that even seemingly odd or lazy behavior has a hidden purpose. Literally: 'a burglar's midday nap (is taken so that he can rob people's houses at night)'.
The fact that he suddenly started studying must be a case of 'a burglar's midday nap'—he's definitely up to something.
A traditional Japanese proverb. The exact origin is uncertain, but it reflects the idea that a thief rests during the day to prepare for nighttime burglary, implying that actions have hidden motives.