The biggest consideration in choosing an alarm clock is usually finding the right buzzer noise -- annoying enough to force you to get up but not so horrible that it ruins your day. Now a bevy of 'smart alarms' have a new selling point: waking you at the perfect time.

A smart alarm can monitor your sleep and wake you gently while you are in a light stage of sleep, which will leave you feeling less groggy, according to the companies that sell the alarms. But sleep specialists are skeptical the alarms will improve alertness, emphasizing instead the importance of a good night's sleep.

Several devices work by 'actigraphy' or monitoring your motion because people are more likely to be moving when sleeping lightly. The alarms determine the optimal time to wake you by sensing increased motion, either by a device that's worn or placed on the bed. You set a window -- say a half-hour -- in which you would like to be awakened.

Alarms designed to wake a person at the best moment have been around since at least 2005, but they were pricey and sometimes complicated to use. The current generation of smart alarms is simpler and less expensive.

Not all devices use actigraphy. The Zeo is a headband with silver electrodes that measure brainwaves and electrical impulses from eye movements. The mobile-device version, which connects to your iPad, iPhone or Android device using Bluetooth, is $99 and the bedside alarm clock is $149. According to Zeo Inc.'s website, its system wakes you gently at the 'optimal point in your sleep cycle,' which the company says is at the transition between light sleep and rapid eye movement, sometimes called REM or dream sleep, according to the Zeo website.

The groggy state after you wake up, in which your cognitive and motor functions are impaired, is called 'sleep inertia,' says Kenneth P. Wright Jr., director of the sleep lab at the University of Colorado in Boulder and a paid consultant to Zeo, of Newton, Mass. Studies have established that sleep inertia is worst after waking from deep sleep, he says, but there is 'some inconsistency' in the literature about when is the best time to wake, for example, during light sleep or REM sleep.

The SleepTracker is a $149 wristwatch alarm that wakes you at the 'true optimum time,' when your body motion suggests you are sleeping more lightly, according to the website of Atlanta-based Innovative Sleep Solutions LLC. You can choose between a vibration, chime or both.

Sleep Cycle is a 99-cent app for the iPhone, which is placed on a mattress. The app analyzes a person's sleep patterns using an algorithm and prompts the iPhone to wake you 'in your lightest sleep phase,' says Sweden's Maciek Drejak Labs AB.

None of the three devices have been tested in clinical studies that prove users are more alert and refreshed using the smart-alarm features. The devices also provide a sleep graph you can analyze, but only the Zeo has proved it accurately distinguishes sleep stages.

In a company-funded study published last year in the Journal of Sleep Research, the Zeo predicted the same sleep cycle as professional sleep equipment in a lab about 75% of the time, compared with 86% agreement for two independent professional lab measures with each other. The device can help you monitor your sleep and test which measures -- such as cutting caffeine in the afternoon -- can improve it, says Zeo Chief Executive David Dickinson.

Some physicians who specialize in sleep are skeptical. 'If I don't sleep enough I'm going to be grumpy. I'm not sure a machine like this will help, and certainly one can't recommend it based on the scientific literature,' says Yale University sleep specialist Meir Kryger. If it does reduce grogginess, he says, the benefit would likely only last 15 or 20 minutes after you wake; at most an hour or two.

The Zeo mobile version was fun and simple to use. I liked the detailed sleep graph it downloaded to my iPad, and it recorded when I was in REM sleep. The headband annoyed me at first and left red bumps on my forehead. After I washed it by hand, per the company's suggestion, to remove manufacturing residue, the problem disappeared.

The SleepTracker generally woke me at the end of the window I had set. Twice, I was already awake. Lee Loree, managing partner of Innovative Sleep Solutions, says the device uses a 'proprietary algorithm' that decides when to wake you. It's likely the device let me sleep longer because it thought I hadn't slept enough, he adds.

To use the Sleep Cycle app, you set the alarm and lay the iPhone face down on your bed. It appeared to sound in the morning as soon as I moved; my cat jumping on and off the bed also triggered it.

Maciek Drejak, lead developer of Sleep Cycle, says the app does record small movements, but when analyzing your sleep patterns, it 'filters' the ones it doesn't think are relevant. He adds the app can't be used to cut back on the sleep you need -- just to wake you up at the right time when you are already rested. The feedback the company is getting from users shows it is helping them wake up in a good mood, he adds.