The short answer for the great majority of boxed foam and hybrid mattresses is that sleeping on it the first night is fine. Many brands say so explicitly in their own setup instructions. A handful ask you to wait a set number of hours before use. The instruction card that came in the box, or the brand's own setup page, is the only source worth trusting on this — the number is not the same for every bed, and third-party articles routinely quote a figure from one brand as if it were universal.
WHY IT MIGHT NOT FEEL RIGHT. A mattress that has not finished expanding is firmer, and often unevenly firm, compared with what it will be in a few days. Foam also needs to reach room temperature to behave normally; memory foam in particular is temperature-sensitive and feels distinctly harder when cold. If you unbox at 11pm in a cool bedroom and sleep on it immediately, you are sleeping on the least representative version of that mattress you will ever experience. That is not damage and it does not mean anything is wrong.
DOES SLEEPING ON IT EARLY HARM EXPANSION? Body weight will not prevent a mattress from expanding, and light use can help it release. What does matter is not putting concentrated point loads on an under-expanded mattress — kneeling on one spot, standing on it, or letting children jump on it — while the foam is still recovering.
THE SMELL, AND WHAT TO DO ABOUT IT. New foam commonly releases a temporary odour when the packaging comes off, from volatile organic compounds trapped in manufacturing and shipping. It usually fades over hours to a few days. Manufacturers' own advice is consistent: unwrap it in a well-ventilated room, open a window, and give it air before you make the bed. Certifications such as CertiPUR-US for foams and GREENGUARD Gold or GOTS for other components exist because emissions are tested; check what your mattress carries if this concerns you. If the smell is strong or you are sensitive to it, air the mattress for a full day in an open room before sleeping on it.
A SENSIBLE FIRST NIGHT. Unbox as early in the day as you can. Unroll it on the base, remove the plastic, open a window, and let it sit for the afternoon with nothing on it. Warm the room rather than cooling it. Make the bed in the evening. That costs you nothing and gives you a mattress much closer to its real feel on night one.
AND THEN BE PATIENT. Do not judge comfort, firmness or your back on the first three nights. Brands write break-in guidance and 30-day-plus trial minimums for exactly this reason, and several return policies require a minimum sleeping period before they will accept a return.