Kural - 672

Slumber when sleepy work's in hand beware
Thou slumber not when action calls for sleepless care

Explanation
Sleep over such (actions) as may be slept over; (but) never over such as may not be slept over.

Tamil Transliteration
Thoonguka Thoongich Cheyarpaala Thoongarka
Thoongaadhu Seyyum Vinai.