Involuntary twitches and apnoea during early phases of sleep
Jerk-like muscle contractions occurring while drifting off to sleep are normal and are referred to as "sleep starts" or sleep myoclonus (National Institute of Health - NINDS). 

Sleep Education explains the symptoms:


  Sleep starts usually consist of one strong jerk that affects much of the body. The arms and legs are most likely to be affected.


So sleep starts may be quite vigorous, but may consist just of small lip movements too.

The American Sleep Association mentions that:


  Sleep myoclonus will rarely disturb the subject or any bed partner to the point of waking and disrupting sleep [...]. 


In fact, sleep starts are associated with tachypnea, i.e., an increased in breath rate (Vetrugno &amp; Montagna, 2011)

Hence, your hypothesis that a myoclonus is a mechanism to wake a person up (for whatever reason) seems unsupported.

Reference
- Vetrugno &amp; Montagna, Sleep Medicine 2011; 12: S11–S16