There's already been a question about devices that can help you monitor your sleep and their possible benefits, but I'm interested specifically in the apps for mobile devices that claim to be able to do this. For this question, I'm not actually interested in if it can be used to benefit your health or make you less "groggy". I'm just interested in whether these apps can actually monitor your sleep cycles to any reliable degree.
These mobile device apps (such as Sleep Time by Azumio or Sleep Cycle Alarm Clock by Maciek Drejak Labs) claim that by placing the phone on your bed near you, they can measure your movement and make some estimates about when you are in a lighter sleep and when you go into deep/REM sleep. I can see how this wold be possible, but I'm doubtful if the results would be at all reliable.
Are there any sources with some data that show that kinds of apps have some merit? Are there any that show they are completely useless? Thank you much!