AbstractObjectiveSleep is vital for normal cognitive function in daily life, but is commonly disrupted in older adults. Poor sleep can be detrimental to mental and physical health, including cognitive function. This study assessed the association between self‐reported short (<6 h) and long (>9 h) sleep duration and sleep fragmentation (3≥ nightly awakenings) in cognitive function.MethodsCross‐sectional data from 8508 individuals enroled in the PROTECT study aged 50 and above formed the basis of the univariate linear regression analysis conducted on four cognitive outcomes assessing visuospatial episodic memory (VSEM), spatial working memory, verbal working memory (VWM), and verbal reasoning (VR).ResultsShort (ß = −0.153, 95% CI [−0.258, −0.048], p = 0.004) and long sleep duration (ß = −0.459, 95% CI [−0.826, −0.091], p = 0.014) were significantly associated with poorer cognitive performance in VWM. Long sleep duration (ß = −2.986, 95% CI [−5.453, −0.518], p = 0.018) was associated with impaired VR. Short sleep (ß = −0.133, 95% CI [−0.196, −0.069], p = <0.001) and sleep fragmentation (ß = −0.043, 95% CI [−0.085, −0.001], p = 0.043) were associated with reduced VSEM. These associations remained significant when including other established risk factors for dementia and cognitive decline (e.g., depression, hypertension).ConclusionsOur findings suggest that short and long sleep durations and fragmented sleep, may be risk factors for a decline in cognitive processes such as working memory, VR and episodic memory thus might be potential targets for interventions to maintain cognitive health in ageing.