Deciding to sleep on a SweetNight mattress or topper the first day won’t damage the material in any way. On the contrary, the body weight and body heat will help decrease the material’s expansion time. Just wait for about 4-6 hours before using the mattress topper to allow off-gassing.
It is safe to sleep on a memory foam mattress or topper on the first day if it has been allowed to expand and settle for at least 4-6 hours. The recommended expansion time for a memory foam mattress or topper is 24-72 hours, so it might feel different after the first night.