About the Other Side of Sleep Disorders: Improving Daytime Problems, Fatigue, Sleepiness, Cognitive Functioning and More.

Daytime impairments-such as fatigue, emotional instability, and cognitive difficulties-are increasingly acknowledged as core features of insomnia, yet they remain underrepresented in both research and treatment strategies. While CBT-I remains the gold standard for treating nocturnal symptoms, its effects on daytime functioning, which are often the primary concern for patients, are less robust, inconsistently measured, and poorly understood. This narrative review highlights the need to elevate daytime symptoms from secondary outcomes to central targets in both research and clinical practice. Key gaps include the lack of standardised and conceptually clear outcome measures, limited personalisation of CBT-I protocols, and insufficient understanding of the mechanisms linking improved sleep to daytime recovery. Moreover, the ethical implications of emerging digital assessment tools must be addressed to ensure that technological innovation does not come at the cost of participant trust or autonomy. To move the field forward, future research should prioritise daytime functioning as a primary endpoint, adopt ethically grounded multimodal assessment strategies, and explore adaptive, symptom-specific treatment designs. Finally, insomnia should be recognised not only as a disorder in its own right but also as a transdiagnostic and potentially preventable contributor to broader mental health problems. Addressing these challenges may lead to more effective, personalised, and patient-centred care for individuals living with insomnia.