ABSTRACT Background Both a diagnosis of prostate cancer and its treatment with Androgen Deprivation Therapy (ADT) can lead to negative side effects on patients' daily functioning, including hot flashes, sweating, and fatigue. Despite this, there is little objective data characterizing the relative contribution of these factors to sleep disturbance, impact on fatigue and quality of life. Aim The aim of this study is to conduct a longitudinal analysis of sleep disturbances in patients with prostate cancer, comparing those undergoing ADT with those who are treatment‐naïve. Methods Subjective data were collected using the Pittsburgh Sleep Quality Index (PSQI) and the Epworth Sleepiness Scale (ESS), while objective sleep assessment was conducted using actigraphy. The study included both ADT and ADT‐naïve patients. Data collection occurred over two timepoints within a 3–4–month period. Clinical parameters, such as nocturia, were also explored in the analysis. Results Actigraphy data analyses revealed that patients who commenced ADT treatment (LHRHa) between timepoints 1 and 2 experienced a significant decline in sleep quality and duration, along with reduced daytime napping. No correlation was found between nocturia and frequent night wakings. In contrast, the ADT‐naïve group demonstrated improved sleep quality and duration at timepoint 2. Conclusion The current study highlights how sleep quality and duration change over time in response to different treatments and their side effects. It provides clear evidence that sleep disturbances in patients on ADT are not temporary but long‐lasting, thereby classifying as chronic insomnia (DSM‐5). Future research should aim to validate these results in larger groups and explore targeted interventions to mitigate sleep‐related side effects of ADT.