Cognitive Behavioural Therapy for Insomnia is the front-line intervention for Insomnia Disorder. However, few studies have reported data on how CBT-I implementation evolves over the course of therapy or the time course of client response. This is due, in part, to no formal framework for defining stages of CBT-I. Here, we follow Spielman's original conceptualisation of Sleep Restriction Therapy (SRT) to report a two-stage model for defining stages of CBT-I, based on progression through the phases of SRT. Using data from a prior randomized clinical trial, we show the model helps identify patterns of treatment progression, which are in turn associated with baseline insomnia severity. The reported model is parsimonious and easy to implement and it holds potential for further examining and understanding a variety of CBT-I processes currently not fully understood, such as adherence and the optimal order of various CBT-I treatment components.