This is a long-term, prospective, observational study to investigate and compare the levels and rhythm of melatonin in patients with AD dementia, mild cognitive impairment due to AD and healthy volunteers. The investigators would like to validate the use of salivary and urine melatonin measurements as an alternative for blood/CSF melatonin. Furthermore, the investigators would like to assess the effects of melatonin levels on cognition by correlating the levels and changes on cognitive tasks over a two year time frame. The investigators will also investigate whether these effects could be due to its anticonvulsive properties.

Melatonin production gets disrupted in AD, as shown in post-mortem pineal glands and CSF of AD patients. CSF melatonin levels are known to significantly drop in patients with Alzheimer's dementia. It is known that CSF melatonin levels are much higher than blood melatonin levels, due to melatonin secretion from the pineal recess directly into the third ventricle. It has never been investigated whether blood melatonin accurately correlates with CSF melatonin in AD, nor whether saliva or urine melatonin levels accurately reflect blood/CSF melatonin in the AD continuum. The investigators want to validate the use of blood, saliva and urine melatonin levels as alternative for CSF melatonin in the AD continuum to pave the way for further use of less invasive collection techniques (blood, saliva, urine instead of CSF) and to possibly study circadian rhythm in a less disrupting, in home environment (saliva, urine).

Furhtermore, melatonin exerts several potential anti-AD properties, including anti-inflammatory, anti-oxidant, tilting APP processing towards the non-amyloidogenic pathway, exerting positive effects on sleep and so on. In vivo studies furthermore point to anticonvulsive and antiepileptic effects of melatonin in a whole range of rodent models. Some evidence exists for a role of melatonin in prevention of epileptic seizures in humans.

The investigators want to investigate influence of melatonin on changes in cognition in a longitudinal way, and investigate influence on (sub)clinical epileptiform activity.

Inclusion Criteria:

1. Dementia due to AD, according to National Institute on Aging-Alzheimer's Association (NIA-AA) criteria
2. MCI due to AD, according to NIA-AA criteria
3. Healthy controls: Age- and gender matched healthy controls

Exclusion Criteria:

Patients (AD dementia, MCI)

• Age < 18 years old
• Pregnancy
• Expected death due to illness within 2 years
• Pacemaker or other ferrromagnetic material that is not MRI compatible
• Other neurodegenerative or cerebrovascular disease
• Pattern compatible with NPH (clinically, imaging)
• Epilepsy
• Multiple sclerosis or other demyelinating disease
• Depression, psychosis or other mental disease
• Use of anti-epileptic drugs
• Alcohol or substance abuse
• Korsakoff syndrome
• Symptomatic liver disease
• Uncontrolled thyroid disorders
• Untreated HIV or syphilis
• Clinically significant vitamin B12 deficiency
• Severe systemic medical illness (eg end-stage cardiac disease, …)
• Use of melatonin, agomelatine, or other sleep medications
• Night worker
• REM sleep behavior disorder, OSAS

Healthy controls

• Age < 18 years old
• Pregnancy
• Pacemaker or other ferromagnetic material that is not MRI compatible
• Mild cognitive impairment or dementia of any cause
• Epilepsy
• Multiple sclerosis or other demyelinating disease
• Depression, psychosis or other mental disease
• Use of anti-epileptic drugs
• Alcohol or substance abuse
• Symptomatic liver disease
• Uncontrolled thyroid disorders
• Untreated HIV or syphilis
• Clinically significant vitamin B12 deficiency
• Severe systemic medical illness (eg end-stage cardiac disease, …)
• Use of melatonin, agomelatine, or other sleep medications
• Night worker
• REM sleep behavior disorder, OSAS