Albert Barnes Commentary 2 Samuel 4:5

"And the sons of Rimmon the Beerothite, Rechab and Baanah, went, and came about the heat of the day to the house of Ish-bosheth, as he took his rest at noon." — 2 Samuel 4:5 (ASV)

Lay on a bed at noon - This refers to taking a midday rest, as is the custom in hot countries.