We have an article from American Journal of Nursing (from year 1966) which proved that the automatic rocking of cradle machine is tested work for irritable and hard to sleep on babies.

This clinical study machine was carry out by a phychiatrist, name Edward Dengrove. The conclusion of the study is that ….

Nearly every baby under three months of age is rocked to sleep within 5 to 15 minutes, provided his other physical needs have been met. The low speed, with the crib oscillating at the rate of about 145 movements per minutes, is usually sufficient for these smaller infants.

Agitated and older babies may take little longer before go into sleep, and a speed of 155 to 165 oscillations per minutes may be more effective. Interesting, oscillation beyond these range are no more effective.

No untoward effect have been noted. This machine have been used with many babies who have had operations for pyloric stenosis. Postoperative vomiting did not increase.

The report also mentions that this oscillating does not seem to be habit forming.

However good that could be. There is one word of caution. The automatic rocker should not be used unsupervised with older babies, who could possibly climb out of the hammock or cribs with ease. It is more suitable for young babies and limited to the age below 2.5 to 3 years old.

Note : The bold text are extracted from the report.