Since I can't see the exact benches you're looking at, I'll give you the realistic range. Most standard park benches have a seat depth of about 18 to 24 inches (roughly 46 to 61 cm). Some older or more minimalist designs run shallower, around 15 to 17 inches (38 to 43 cm), while a few oversized or custom benches can go deeper. But the short answer? Depth alone makes a bench borderline for napping—it can work for a quick rest, but it's rarely a comfortable bed replacement.
If you're just sitting, a depth of 16 to 20 inches feels normal because it supports most of your thigh without digging into the back of your knees. When you lie down, however, the same depth has to support your shoulders, torso, and hips all at once. On an 18-inch seat, that means a good part of your body is either balanced on the edge or pressed against the backrest. If you're a smaller person or you sleep on your back with your arms crossed, you might manage. If you're taller or you move in your sleep, you'll probably feel like you're one breath away from rolling off.
The bigger issue is usually the bench's layout, not just depth. A lot of park benches have armrests dividing the seat into sections. Those armrests can make a nap almost impossible unless you curl up tightly in one section or accept that your legs will hang over the armrest. A flat, armrest-free bench with a depth of 20+ inches is your best bet. Length matters too: look for something at least 60 to 72 inches long if you want to stretch out. Wooden slat benches and metal benches are also harder than they look. They don't cushion your hip or shoulder, so even 20 to 30 minutes can leave you sore in bony spots.
I've tried napping on a 20-inch-deep wooden park bench a few times. On my back, it was doable for about 25 minutes. I used a backpack as a pillow and kept one leg bent with my foot on the seat and the other foot on the ground to stay stable. On my side, it was much harder—the edge hit right at my hip, and my arm had nowhere to go. If the bench had been 15 inches deep, I would have given up immediately. If it had been 24 inches with no armrests, I could see it being fairly usable for a short nap.
So is it suitable for napping? For a light 20- to 40-minute nap, yes, many park benches can work if you pick the right one and accept the discomfort. For anything longer or deeper, probably not. You'd be better off on a blanket in the grass, a hammock, or a bench with a padded seat and no dividers. If a park bench is your only option, choose a wide, armrest-free bench, lie on your back, use a bag or jacket as a pillow, and keep a hand on your belongings. It's not a bed, but it can be a decent emergency nap spot.