The stress hormone cortisol is the end product of the hypothalamic–pituitary–adrenal (HPA) axis, and the cortisol awakening response (CAR) refers to the rapid rise in cortisol levels observed immediately following awakening. During the CAR period, cortisol levels typically increase by 38%–75%, peaking approximately 30 min after awakening. Evidence suggests the function of the CAR may be related to arousal, energy boost and/or anticipation, although its precise function is still unknown. The CAR has been investigated in a range of clinical populations including the assessment of daytime dysfunction in insomnia; however little research, if any, has specifically examined its relation to sleep architecture, or night-time difficulties associated with insomnia. The aim of this review is to provide an overview of the CAR, a description of the factors which can affect it, and to outline the CAR in relation to the ‘3P’ model of insomnia. This review concludes with a description of a standard protocol for measurement of the CAR, which can be adapted and applied within sleep medicine.