Every night Jack Hammond straps a small gadget to his wrist. The sensor claims to track his sleep, monitoring every toss and turn so that when he wakes it can tell him how long he slept, how much of it was deep sleep – and how often he was woken up by his unsociable neighbour.

"I can see that I need about four hours' deep sleep to feel refreshed, and I need to be asleep for about eight hours for that to happen," says Hammond, a 36-year-old plumber from Northhampton. "I can also see that when I've had caffeine too late I get less deep sleep."

Hammond's gadget is a fitness tracker – a small bracelet device that by day can measure his activity and calorie burn, the distance he has walked and altitude climbed, and reveal how long he spends at his desk. Part of the "quantified self" movement, more than 3.8m fitness trackers like Hammond's were sold worldwide in 2013, predicted to rise to 14.6m by the end of 2014, according to the analysts CCS Insight.

"Several devices have supported sleep measurement for some time, from smaller companies such as Jawbone, Fitbit and Basis," said Ben Wood, the head of research at CCS Insight. "The big players are now getting in on the action and Sony and Samsung devices support sleep tracking too."

But do the devices work? Does using them do you any good?

Sleep has always been considered scientifically important to all-round physical health. But efficacy of fitness trackers to measure and improve sleep is less established.

Studies have found that people who sleep for less than six hours a night have a risk of high blood pressure three times greater than those who get more than six hours, and that women who sleep less than four hours a night are twice as likely to die from heart disease as those who sleep longer. Other research suggests that a lack of quality sleep is linked to the onset of diabetes, obesity and cancer, not to mention deterioration of mental health and memory. Conversely, sleeping too long has also been shown to cause issues. The recommended amount is between seven and nine hours.

"There's physical restoration component to sleep, including healing, as well as mental restoration component," explains Dr Michael Breus, a US "sleep doctor", based in Scottsdale, Arizona. "During REM [rapid eye movement] sleep in particular there's a cleaning process to get the trash out of there, and a strengthening process that occurs for the more important memories," said Breus.

"Anything that allows you to wake up feeling refreshed in the morning is what you should be aiming for, which is generally between six and nine hours," explained Dr Irshaad Ebrahim from the London Sleep Centre. "But it's not all about time, it's about the quality of the sleep, whether you complete sleep cycles."

As to using motion trackers to measure sleep, he said: "They're not measuring sleep, simply motion – not muscle tone, brain waves, heart rate or eye movement. You cannot infer quality of sleep from motion and tell what is crucial REM [rapid eye movement] sleep and what is not. People can become obsessed about their sleep through these gadgets doing them a disservice, worrying about it and in turn getting less decent sleep and having a negative impact."

Breus is less concerned. "They pique people's curiosity, and it gets them to ask 'how is my sleep'? That's the best thing about them. I wouldn't say they are dangerous, but it's a garbage-in, garbage-out situation, and it's impossible to make recommendations without good data."

For some users, that curiosity can lead them to understand how to sleep better. "My Fitbit told me I slept for more hours than I thought I did," said James Stockton, 29 from Southampton. "It was quite encouraging, and I could see patterns in my sleep when I exercised more or drank alcohol."

If, however, inspired by a sleep gadget, you want to find out general tips for better sleep, you will discover the advice doesn't fit so well with a modern lifestyle. Screens on smartphones, tablets and TVs emit blue light that disrupts the natural process of falling asleep – and is best avoided for an hour before going to bed. Avoiding alcohol and caffeine raises the quality of sleep, as does getting regular exercise.

Motion trackers aren't the only gadgets claiming to track sleep. Products such as the SleepRate, which comes with a chest strap, will track heart rate, which can be used to infer sleep. Although more accurate than motion trackers, they still cannot conclusively monitor all stages of sleep and reveal the crucial quality of a period of sleep.

"I get patients showing me their sleep data on smartphones," said Breus. "But I can't tell them what I don't know. The data isn't good enough to give them a diagnosis. They could be useful for tracking sleep trends over a longer period of time, to see when something changes, though."

Most of the devices currently available to consumers simply track motion to infer sleep. Sleep therapists use much more sophisticated machines that monitor brain waves, muscle tone and eye movement to directly detect the different phases of light and deep sleep.

The next wave of sleep gadgets, expected to hit the market within six months, is likely to include more sophisticated sleep-trackers that can monitor brain wave activity and eventually other biometric indicators, such as heart rate, muscle tone, eye movement and breathing rate.

Brain wave patterns are thought to be the best indicator at present. "We expect this to become a standard feature in most wearable devices , both fitness bands and smart watches, from now on," said Wood.

In the meantime, many sleep-tracking devices are built into tools for tracking more general fitness – and there is evidence that consumers are losing enthusiasm. A third of owners abandon their fitness tracker within six months, according to research from Endeavour Partners.

"I used to track my sleep with my Flex, but I gave up in the end," said Jessica Ross, 22, from Liverpool. "It was boring and didn't really tell me anything to keep me wearing it either at night or through the day. It lasted four months and then went in a drawer."

Top trackers

There are many devices on the market that claim to track your sleep, some better than others.

Here are six of the best available at the moment.

Misfit Shine – £80 A small metal waterproof disc, the Misfit Shine is one of the smallest and most comfortable to wear. It automatically detects deep and light sleep and awake moments via movement. It has a four-month battery life and outputs the data via Bluetooth to an Android or iPhone app.

SleepRate – £60

Combining a chest heart rate monitor with a smartphone app, SleepRate, which uses a heart rate strap, monitors sleep and can craft a bespoke sleep improvement plan. SleepRate claims to be more accurate than motion-based sleep monitors and if serious problems are detected SleepRate will suggest a visit to a sleep doctor.

Jawbone UP24 – £125 This fitness band can monitor light and deep sleep and waking periods using movement while automatically connecting to an iPhone or Android smartphone via Bluetooth. It will also wake up the wearer with a small vibrating alarm at the optimum time for a maximum refreshed feel.

Fitbit One – £80 Fitbit's pedometer-style fitness tracker can also monitor sleep, slipped into a small soft band worn around the wrist. Like the others it detects movement and can wake the user through a small vibrating alarm.

Fitbit Flex – £80 The Flex is essentially the Fitbit One formed into a bracelet, detecting sleep through moment and connecting to the Fitbit iPhone, Android and Windows Phone app via Bluetooth.

Withings Pulse O2 – £100

The Pulse will track sleep via movement, giving readings on total sleep and sleep cycles, slipping into a bracelet worn around the wrist. It connects via Bluetooth to an iPhone or Android app to display the data.