For Lisa Hanson, a stay-at-home mother in San Francisco, the alarm that used to rouse her out of sleep most mornings was the sound of her toddler.

"He wakes up at 5:30 screaming sometimes," she said. "It would jolt me awake, and then I'd be miserable and groggy all morning because I'd be woken up when I'm in my deepest sleep."

But last month, Ms. Hanson started using a new program, the Renew SleepClock, an iPhone app that keeps track of her sleep. Its makers, GEAR4, say that the program uses radio sensors -- built into a bedside device the iPhone is plugged into at night -- to detect breathing patterns and movements. It then uses that information to wake a person at the lightest point of sleep, the optimal time to wake up. The theory is that awaking from light sleep, as opposed to the deep stages of sleep, helps reduce so-called sleep inertia, the cloud of grogginess and impaired alertness that makes people desperately want to crawl back into bed. The app also acts as a sort of sleep adviser, giving Ms. Hanson guidance about the amount of shut-eye to shoot for and ways to get there.

"Since I've been using it, I do feel better," Ms. Hanson said. "It's waking me up in a more ideal stage of sleep and helping me have that much better of a day."

The Renew SleepClock, which costs $199, is the latest addition to a new generation of smartphone apps designed to analyze and improve sleep patterns. While experts have warned for years that gadgets like smartphones are increasingly disrupting sleep by keeping us connected 24/7, these programs claim to do the opposite. Two other products that have gained followings are the WakeMate ($59.99), a wristband worn at night that wirelessly transmits data to the user's smartphone, and the Zeo Sleep Manager-Mobile ($99), which uses a sensor-equipped headband to collect data about the user's sleep habits.

While all the apps vary, they generally work by having users pick a window of time in the morning - typically 20 to 40 minutes long - during which they would like to get up, then nudging the sleeper awake at the lightest point of sleep in that window. Sleepers get a score each morning that factors in the number of hours they slept while adding and subtracting points for things like disruptions and the percentage of time spent in the deep, restorative stages of sleep. The programs also let users keep track of daily habits like caffeine intake, exercise and meals, among other things, so they can see which habits might be affecting their sleep.

But while some people swear by these tools, scientists caution that the claims they make and the advice they dole out to users may not be very sound.

"I think these are clever devices, but there's very limited data on how valuable they are," said Dr. David M. Rapoport, the director of the sleep medicine program at New York University School of Medicine. "They're a really good way to capture data. But some of them are giving out recommendations in ways that just aren't proven."

He points out that certain features, like quantifying sleep and tracking habits, are particularly beneficial because they identify sleep patterns. "Many people lie to themselves about how much sleep they're really getting," he said. "If it takes an app to tell you you're only getting four hours a night and that that's why you're tired, then that's a good thing."

But, he says, when an app instructs you to get eight hours of sleep instead of seven, or to avoid coffee after 6 p.m. for better sleep, it has gone too far, since not everyone needs the same amount of sleep or is affected in the same way by caffeine. "These apps tend to suggest that there's a right way you should do things," he said, "but we cannot make the same recommendations for everybody."

Dr. Rapoport also questioned the claim that waking a person from light sleep prevents sleep inertia. While some studies have found evidence of this, others, like one carried out by Harvard researchers and published in The Journal of Sleep Research, have found it has almost no effect. "It is not clear that systematically waking people up in light sleep is going to benefit them or lead to better health," he said.

Toward the end of a normal night of rest, the amount of time spent in deep sleep tends to diminish, so it's likely that any alarm will wake a person in light sleep anyway, said Dr. Meir H. Kryger, a professor at Yale Medical School and the author of a new e-book, "The iGuide to Sleep."

"At 6 or 7 in the morning, it's very likely that a person will not be in a very deep state of sleep," he said. "Simply knowing that means that these devices have a very good chance of working just because of biology. On the other hand, so does any alarm clock."

That is not enough to dissuade faithful users of the programs, like Bart Epstein, 43, a longtime Zeo user who lives in Arlington, Va. Mr. Epstein, an executive at Tutor.com, an online tutoring program, says he has no doubt that the program lives up to its claims. After years of sleeping poorly, he began using the Zeo, which helped him identify habits that were keeping him "wired" at night and cutting into his sleep, like evening exercise, and it forced him to be disciplined about his bedtime regimen.

"Having all the data that it collects is what influences me," he said. "You can lie to yourself and lie to your spouse, but you can't lie to the Zeo. It holds you accountable."

Best of all, he said, he no longer struggles with morning grogginess. "It just doesn't happen anymore."