Nighttime diaper means: An extra-bulky and absorbent diaper which is intended to be worn during sleep for prolonged wetting, usually by an infant who is a heavy wetter, or by someone with urinary incontinence.. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.