FAQ THE MATTRESS WHEN CAN I SLEEP ON MY NEW MATTRESS? SEE ALL QUESTIONS WHEN CAN I SLEEP ON MY NEW MATTRESS? You can sleep on your new mattress the night you get it! In some cases it may take your mattress up to 24 hours to expand to its original height, but it will not damage the mattress to sleep on it before then. YOU MAY ALSO LIKE Does the foam smell? What certifications do Casper mattresses have?