Under a Merlin Magic Sleepsuit, your baby should wear a diaper and a thin cotton onesie. That’s it. The suit itself is already a thick, padded layer designed to muffle the startle reflex, so adding more clothing underneath increases the risk of overheating.
The Basic Rule: Onesie or Diaper Only
The manufacturer recommends dressing your baby in either a single-layer cotton onesie with a diaper or just a diaper underneath the suit, with the room kept between 68 and 72°F. Which option you choose depends on two things: which version of the suit you have and how warm your baby’s room runs.
The Merlin Sleepsuit comes in two fabrics, and each one changes the equation for what goes underneath:
- Microfleece version: Thicker and warmer, better at muffling the startle reflex. In a room at 70–72°F, a diaper alone underneath is often enough. If your room sits closer to 68°F, a thin short-sleeve onesie works well.
- Cotton version: More breathable and flexible. A long-sleeve cotton onesie underneath pairs well with this version in rooms at 68–70°F. In warmer rooms, stick with a short-sleeve onesie or just a diaper.
Both versions have open hands and feet, which helps heat escape. This is intentional. Don’t add socks, mittens, or hats, as these block your baby’s main ventilation points.
Adjusting for Room Temperature
The AAP recommends keeping your baby’s sleep space on the cooler side and dressing them in only one more layer than you’d be comfortable wearing. Since the Merlin suit already counts as a significant layer (it’s padded, after all), the clothing underneath should be minimal.
Here’s a practical breakdown:
- Below 68°F: A long-sleeve cotton onesie under the suit. Consider whether your room is too cold and adjust the thermostat rather than adding more layers to the baby.
- 68–70°F: A short- or long-sleeve onesie, depending on whether you’re using the microfleece or cotton version.
- 70–72°F: A thin short-sleeve onesie or just a diaper, especially with the microfleece version.
- Above 72°F: Diaper only, and strongly consider switching to the cotton version if you haven’t already. Rooms above 72°F with a microfleece suit can get too warm quickly.
What Not to Put Under the Suit
Skip footed pajamas, fleece layers, sleep sacks, or anything with extra padding. The whole point of the Merlin suit is that the suit itself provides the bulk. Layering another thick garment underneath defeats its purpose and creates a real overheating risk. A simple cotton bodysuit is the thickest thing that should go under it.
Also avoid hats or head coverings. The AAP specifically advises against putting hats on babies indoors once you’re home from the hospital. Babies release a significant amount of heat through their heads, and covering it during sleep can cause dangerous temperature buildup.
How to Check if Your Baby Is Too Warm
The back of your baby’s neck or their chest is the most reliable spot to check. If the skin there feels hot or damp, your baby is overdressed. Don’t go by hands and feet, which naturally run cooler in infants and aren’t a good indicator of core temperature.
Signs your baby is overheating include flushed or red skin, sweating or damp hair, unusual fussiness, and sluggish or listless behavior. Heat rash, which looks like tiny red bumps in skin folds around the neck and diaper area, is another common signal that your baby has been too warm. Keep in mind that babies can overheat without visibly sweating, so touch-checking their skin regularly during the first few nights with the suit is a good habit.
Overheating during sleep is a known risk factor for SIDS, which is why getting the layering right matters more than just comfort.
Fit Matters for Safety Too
Whatever you put under the suit, make sure the suit itself still fits properly. The small size is designed for babies 12 to 18 pounds (roughly 3 to 6 months), and the large fits 18 to 21 pounds (6 to 9 months). A bulky underlayer that makes the suit fit tighter than intended can restrict movement or make the suit ride up.
The suit should be snug but not tight, with your baby’s hands and feet emerging freely from the openings. If you notice the suit pulling at the seams or your baby’s toes curling against the foot openings, the issue might be sizing rather than what’s underneath, but extra clothing can make a borderline fit worse.
When to Stop Using the Suit Entirely
The key safety milestone is rolling. Once your baby can roll over while wearing the Merlin suit, it’s time to stop using it. This is different from rolling in regular clothes. The suit restricts movement, so the relevant test is whether your baby can turn over with the suit on. If they can, the suit no longer provides the safe, back-sleeping position it’s designed for.
You should also stop if your baby exceeds the weight limit for their size. The company makes a transition product (the Magic Sleep Sack) for babies who’ve outgrown the suit, which uses a thinner design and allows more movement. At that point, the layering rules change completely since a sleep sack is much less insulated than the padded suit.