A few weeks ago I was having a great deal of trouble falling asleep. Nervous about teaching 9 sessions in two days to a completely new group of students, I needed sleep, but it eluded me.
One of the yogic techniques I tried was echo breathing. I’d read about this type of breathing in B.K.S. Iyengar’s Light on Life. Here’s how Mr. Iyengar describes how to do it and what it achieves:

“Exhale slowly and fully. Pause. Then exhale again. There is always a slight residue left in the lungs. In that residue is to be found the sludge of toxic memory and ego. In that brief further exhalation, let them go – and experience an even deeper state of relief from burden, of peace and emptiness. In inhalation we experience the full ‘I’, human potential fulfilled and realised…. In exhalation we experience the empty ‘I’, the divine void, a nothingness that is complete and prefect….”

Did the breathing put me to sleep? Well, unfortunately not. If I’d been able to stay with the cycles, it might have. But that mindstuff – citta vrtti – continuously shoved its way in and hijacked my attention to breathing. Eventually, my exhausted mind did just give itself up to sleep.
I’ve heard it said that the way it works is that pausing at the end of inhalation or exhalation creates a kind of gravitational pull on consciousness, focusing it inward, away from distraction.
It may not necessarily work as a sedative, but I did have plenty of energy for my heavy teaching load the next day, even with minimal sleep.

Prachchhardana vidharanabhyam va pranasya

Or by pausing after breath flows in or out.*
*The Yoga Sutra of Patanjali, translation and commentary by Chip Hartranft