Sleep tech
Two categories dominate consumer sleep hardware: things that measure your sleep, and things that change it. They are usually reviewed together and shouldn't be — the evidence behind them is completely different.
Measurement devices (rings, straps, watches) infer sleep stages from pulse, motion and temperature. They are accurate for sleep timing and heart rate, approximate for staging, and change nothing on their own. Intervention devices (bed-cooling systems, mattress covers) act on a mechanism with strong physiological support — core temperature has to fall for sleep to begin — and their value depends on whether you are actually too warm at night. Buying a tracker to fix bad sleep is a category error. Buying a cooling system when you already sleep cold is an expensive one.
Measurement
Trackers and rings
Smart ring buyer's guide
Four questions decide this purchase, and none of them is "which has the best sensors." Form factor, subscription model, what you'll do with the data, and whether you'll still wear it in month six.
Read the guide → ComparisonOura Ring 4 vs Whoop 5.0
$349–499 hardware plus $5.99/mo against $199–359/yr with the hardware included. The pricing structures push you toward completely different commitments.
Read the comparison →Intervention
Bed temperature systems
Bed cooling systems explained
Water-circulating covers, air systems and passive materials do genuinely different jobs. What the specifications leave out, and how to work out whether you need one at all.
Read the guide → ComparisonEight Sleep vs Chilipad
Automation against ownership. One adjusts itself all night and bills you annually; the other holds the temperature you set and asks for nothing more.
Read the comparison →Before you buy
Three things worth knowing about this whole category
The subscription is usually the real price
Sticker prices in sleep tech are increasingly a down payment. A Whoop membership starts at $199 a year and the hardware is bundled into it, so year five costs the same as year one. An Eight Sleep Pod 5 Ultra starts at $4,849 and then adds an Autopilot plan from $199 to $299 a year for the features the product is marketed on. Oura is the mildest case at $5.99 a month, but it still gates the app behind it. Every comparison on this site prices five years, not one.
Sleep staging is an estimate, and the good news is it's a decent one
A study run at Brigham and Women's Hospital put a consumer ring at 79% agreement with clinical polysomnography on four-stage classification. For scale: two human technicians scoring the same night independently agree about 83% of the time. So the gap between a good consumer device and a sleep lab is smaller than most sceptics assume — and also smaller than the marketing implies, because that 79% is the ceiling, not the average across the category. The full picture is in tracker accuracy.
Temperature has the strongest evidence of anything you can buy
Core body temperature must fall roughly 1–2°F for sleep to begin, and a cool room accelerates that drop. This is settled physiology, not a wellness claim, and it is why bed-cooling systems have a more defensible rationale than almost any other product in the category. It is also why they are useless to people whose bedrooms are already cold. The mechanism is explained in sleep and temperature.