you can build a VERY SIMPLE heart rate monitor by using a photodiode and a led… basically, when your heart beats, blood under your skin increases and decreases… these cause the skin to be more or less opaque. This is very very cheap to do but means the device has to mentain a constant distance from the skin, which can be hard when you’re sleeping (then again, you can encase it in a small box that pushes against the sking)…
the second way is by measuring electricity across the skin, electrodermal style (like the GSR)… there are long term fluctuations (during a week for instance, your level of activity across the skin can change), then there are some other large fluctuations, which are linked to stress levels, sleep, etc and smaller fluctuations, which happen cause of the heart beat… that’s how most of those watches work.
now, i did a research on heart beat and while it’s true that it changes during REM the change is not that significant, it’s (from what i read) about 5 to 10 beats per minute differece, which may not be a strong enough cue for an rem detector…
but since we’re talking straps, BREATH changes SIGNIFICANTLY during sleep. if one could measure breath frequency and amplitude (both of which change) you’d have a very very good means of detecting REM which could be somewhat cheap… i just don’t know how those breath straps work though.
and, the second thing we though would be a good way of detecting REM was of course GSR (electrodermal that i mentioned before), which would be transferred now to sound.
the idea is instead of complicating ourselves with analog to digital devices, we can just use the same thing the GSR 2 has, a line out. You plug it in the computer and you basically find the frequency of the sound (which would be directly proportional to the electrodermal responses) and there you have it… then you use a cheap pair of glasses like this to signal rem.
the advantage is the cost, and the fact you could program the glasses to put you to sleep and signal when necessary, like the Dream Maker PRO and VERY EXPENSIVE… not to mention additions of sound, biofeedback, etc… it would mostly be software driven.