Countless insomniacs yearning for sleep have looked with envy at their cats, wishing they too could just curl up anywhere and nap. Turns out the cats may hold the secret to natural, drug-free sleep. A report to be published Friday in the journal Science says it may be possible to synthesize a brain chemical found in cats, but possibly common to all mammals, that leads to peaceful drowsiness. If so, they may be able to produce a new sleeping aid that does not carry the usual "hangover" side-effects of grogginess, headache and exhaustion. Scientists at the Scripps Research Institute in La Jolla, Calif., made the discovery by putting cats on a moving treadmill and depriving them of sleep for up to 18 hours, then studying changes in their chemistry once they drifted off to dreamland.