The reading for this word is an exception. Here's a mnemonic to help you remember it:
If you're feeling sluggish just turn on Darude's (だる) "Sandstorm" and it will perk you right up. But a world where people don't know Darude's "Sandstorm" is full of sluggish, dull nerds.
My body feels heavy because of my high fever from influenza.