Jawbone's Up fitness trackers have been spotted on the wrists of celebrities such as Gwyneth Paltrow, Kristen Stewart and Bryan Cranston.

Up's beautiful and sleek design makes the devices look more like a fashion accessory than a piece of gadgetry. And Jawbone's latest Up3 fitness tracker is no different.

Designed by the renowned Yves Behar (Up2 and Up24 were also his work), the newest addition to the family has a slim anodised aluminium casing with a rubber wrist strap.

It comes in silver or black. The silver one has a twisted cross design on the casing with grey straps, while the black one (the unit being reviewed) has diagonal lines running across the top of the black casing, and matching rubber straps

Up3 looks much like Up2, which was reviewed recently. Up3 is 0.7mm thicker and 4g heavier and, like Up2, has an adjustable clasp to secure it to your wrist.

The clasp is not very secure. It came undone and my Up3 slipped off my wrist while I was extracting a credit card from my wallet.

The biggest difference between the older devices and the newest are Up3's four tiny gold-coloured metallic square tabs on the inside of Up3's straps and the single tab on the underside of the casing.

These tabs are bio-impedance sensors that automatically track your resting heart rate.

The Up3 was supposed to be waterproof. But just before its release, Jawbone decided to rate the fitness tracker as only splash-proof.

In other words, it is safe to wear Up3 when you wash your hands or take a shower, but take it off before you go swimming.

To start using Up3, you need to pair it with the new Up app (Android and iOS) on your smartphone. You need this new app to pair with Up2 and the upcoming Up4. The older Up app can pair only with Up24 and the original Up.

Confused?

You won't be the only one.

One irritant is the need to place Up3 on the USB charging cable to be paired. At times, the Bluetooth connection failed and I could not pair the devices again because I did not have the charging cable with me. Otherwise, pairing is a breeze. The app detected the Up3 and paired within a minute. The next time, all you have to do is launch the app and it will sync automatically with Up3. Do remember to update Up3 to its latest firmware.

Like Up2, Up3 has no display. On its casing are three LED icons: a jogging man to indicate awake mode, a moon icon for sleep mode and a message icon to indicate notifications from the Up app. It is, by default, in the awake mode. The device will track if you have been on a run or long walk and the app will ask if you want to log the activity.

But you must switch it to sleep mode before going to bed. To do so, double tap on the aluminium casing, then hold it down until the band vibrates and the moon icon appears. This type of sleeping timing was fairly accurate.

Even if you fail to do so, Up3 will detect your ''inactivity'' and ask if you have slept within a certain period. But you will not be able to tell what sleep phases you have experienced. Otherwise, the app will tell you how long you spend in sound sleep and light sleep, as well as how long it takes you to fall asleep. It even detects how many times you wake up.

I found the waking-up time to be in line with the times I woke to go to the toilet or for a glass of water.

On the downside, the Up3 does not track your heart rate constantly, but tracks only the resting heart rate which it calculates while you are asleep. It would have been useful to know how my heart performs during workouts.

Fitness statistics, such as steps taken and distance travelled, tracked by Up3 are quite accurate. Compared with my calibrated Apple Watch, there was only a 2.5 per cent difference in readings.

Other Up3 functions include Smart Sleep vibrating alarms, which will wake you without disturbing your spouse. You can also set Idle Alerts so that the device will vibrate to tell you if you have been sitting for too long.

The advertised battery life is seven days. But by the end of six days, the app had begun reminding me to charge the device. Still, the battery life is quite good.

Up3's biggest drawback is its hefty price tag. At $309, it costs $110 more than Up2 and the Fitbit Charge HR.

And Charge HR has a small display and can track your heart rate constantly, though it is certainly not as good-looking or as fashionable as Up3.

•If you are looking for the bestlooking fitness tracker with a heart rate sensor, the Jawbone Up3 is your pick - especially if you are willing to pay for it.