My sleep-cooling unit died, and the replacement cost more than I'd pay for a pump pushing cold water through a mat. A current Chilipad runs €1,600–1,800; an Eight Sleep Pod €2,500–2,850, several with an annual subscription without which paid features switch off. I built the equivalent for €350–530, or €150 if you already own the one part that matters — the pad. Everything else just makes cold water and moves it.

Why bother

To fall asleep your core temperature has to drop a degree or two, shedding heat through your skin. A mattress traps that heat, and cooling the surface fixes it. But more cooling isn't better: drive the surface too cold and skin vessels constrict, which cuts the heat loss that triggers sleep onset and wakes you cold. The right target is the one you find by experiment — mine is around 18 °C water.

The build

Instead of an aquarium chiller, the cooling source is a portable compressor cooling box, the camping kind. It both holds the water and cools it: the interior is a watertight shell, so you pour water straight in, and the wall sits against the evaporator as a large heat exchanger. A small pump circulates water out through a hose, into the pad, and back in a closed loop. The only modification is one drilled hole for the hoses and pump cable, sealed with silicone — mainly against condensation, not for thermal gain.

Mine uses an Alpicool C15 (15 L, ~45 W) and a 12 V submersible pump. The water-directly-in-the-box arrangement is the strong part: direct coupling, large surface, no intermediate coil. If you own a dead branded unit, keep its pad and replace only the guts; from scratch you also need a pad.

It's easy. Nothing soldered, no wiring beyond plugging in, one hole drilled. The only tool is a drill.

How much cooling reaches you

An order-of-magnitude estimate, not a precise figure. A box drawing ~45 W produces, through the refrigeration cycle, roughly input × COP of cooling; for cheap DC compressor boxes the COP is around 1.3 at this small temperature lift, so ~55–60 W gross. From that you lose what never reaches you: a few watts through the walls, 10–15 W from the pump (a submersible dumps its whole draw into the water), and 5–15 W picked up by the hoses depending on length and insulation. That leaves about 35–40 W removing body heat in a normal room, nearer 30 in a hot one.

A single sleeper sheds roughly 30–60 W through the pad contact area. So a small box sits at the lower edge of sufficient: fine for one person in a normal room with little margin, tight on a hot summer night — the same league the branded units actually deliver. The bottleneck is compressor capacity against the lift plus hose pickup, not the box-to-water transfer. If you can't hold temperature in summer, insulating the hoses beats a bigger compressor.

The pump tax

That 10–15 W of pump heat is a fixed fifth-to-quarter of your budget, paid nightly. The structural fix is an external inline pump: with the motor out of the water its losses go to air, leaving only the hydraulic work in the loop, which roughly thirds the penalty. The cost is complexity — priming, bleeding, more leak points, and it must be quiet. Cheaper habits help too: don't over-pump (hydraulic power climbs steeply with flow past the rate that gives good transfer), and use wide, low-restriction tubing. A more efficient pump matters less than getting it out of the water.

Pre-cooling: the water as a thermal battery

The water mass stores cold. Run the box for ~2 hours before bed and pre-chill below your target — say 14 °C when you want 18 °C at night. Through the night the compressor runs flat out, and the shortfall between demand and its output is drawn from the banked cold as the water drifts back up. Roughly 10 kg pre-cooled by 4 °C banks ~46 Wh; against a ~12 W shortfall that's about four hours, stretching toward six with more water or a deeper chill.

Three things line up: deep sleep is front-loaded into the first cycles, so the buffer covers the window that matters most; room temperature usually falls overnight, so demand drops as the buffer empties; and the gentle warming toward morning is the right direction anyway, easing you toward waking. The limit is depth — stay within 3–5 °C below setpoint or the pad is too cold at lights-out.

When you need a bigger box

If you're capacity-bound in summer, go larger. An Alpicool GE50 (50 L, 60 W input, ~€191) produces ~78 W gross, leaving ~48–57 W at the body — clearing summer with margin. The 50 L also lets you pre-cool enough water to cover essentially the whole night. Downsides: a large footprint by the bed, a louder compressor (longer hoses, site it away), and ~140 W of waste heat into the room, so the condenser wants fresh air. One label trap when shopping: check whether a box's wattage is input or cooling capacity — they differ by the COP. The GE50's 60 W is input.

The cost ladder

The pad is the unavoidable variable; you can't always buy it alone, so you either harvest one from a cheap complete device or buy a replacement topper. Around that:

  • Own a pad (~€150): box, pump, power supply, tubing, ~€10 timer plug.
  • Proof of concept (~€350): small box, cheap submersible pump, harvested pad. One person, normal room, tight in summer.
  • Daily driver (~€430): same with a proper replacement pad. Identical cooling, a pad that's actually comfortable. The sweet spot for most.
  • Summer-robust (~€510): GE50 plus a proper pad. Capacity headroom and a whole-night buffer.
  • Max / two sleepers (~€530): add a quiet external pump. Diminishing returns.

Two axes: pad quality and summer capacity. Stacking both capacity upgrades partly cancels — the big box removes the need to fight the pump tax — so the top tier isn't the default. Most land at the daily driver. Even the top build is about a third of the cheapest branded unit.

Control and safety

A €10 timer plug does most of what the expensive units charge for: switch the box off 30–60 minutes before your alarm and the bed warms as you wake, which is the natural cue. Some boxes add app scheduling; the timer works regardless.

Water and electricity share a bed here, so the safety basics aren't optional: a GFCI outlet, a waterproof protector under the pad, a leak test before you sleep on it, and regular cleaning of the loop.

Who it's for

Optimizers, hot sleepers, and anyone minimally willing to tinker who'd rather not pay four figures to replicate basic thermodynamics. If you want a sealed appliance you never think about and money is no object, buy the branded one.