You might be tempted to let your baby fall asleep in yor Pack'n Play. That depends on what you mean by “mattress”.  Each Pack ‘n Play comes with a mat or mattress of some sort. Allowing your baby to sleep on the mat that it comes with is perfectly acceptable, although it may not be as comfortable as allowing your baby to sleep on an actual mattress. If you do opt to get a mattress for your Pack ‘n Play, make sure that it is the correct fit. A standard crib-size mattress will not fit a Pack ‘n Play. Moreover, there are risks involved with replacing a Pack ‘n Play mat with a soft mattress. If your child is under the age of one, it is recommended that your child sleeps on a surface that is flat and firm. Never, under any circumstances, should you remove the mat or mattress that your playard came with to allow your baby to sleep on the bottom of an uncovered playard. Doing so is not only extremely hazardous but will be highly uncomfortable for your baby or toddler.