Earlier quoted context omitted.
I wouldn't start CBT-I without first getting a sleep test, particulary for sleep apnea which can be present without obvious symptoms beyond insomnia or a delayed sleep phase. It's also much more common in young fit people than is often thought.
I would by virtue that CBTi isn't just sleep restriction, the most important component is CBT. At any rate restriction need not be very severe.
My point is that CBTi isn't what you need if you have sleep apnea or some other treatable disorder as the root of your sleeping issues.