Since initial description of sleep attacks at the wheel in patients with PD treated with dopamine agonists [28], many reports have focused on excessive daytime sleepiness (EDS) in patients with PD but the best method to explore EDS in PD remains unclear. The subjective nature of EDS complaint, the definition used, the different populations targeted, the potential confounding factors, and the various methodological tools used to evaluate sleepiness explain most of the discrepancies between studies on EDS prevalence and risk factors in PD.

Multiple sleep latency test (MSLT) is the gold standard to objectively explore sleepiness in central hypersomnia [29]. Several studies have recorded nighttime sleep and MSLT to objectively measure hypersomnia in patients with PD [30–34]. Surprisingly, none of the controlled studies found significant differences on the mean sleep latency on MSLT between unselected patients and controls [30, 32, 33]. A narcoleptic-like phenotype was also reported in unselected patients with PD [30, 31, 33]; this propensity increased when patients were selected for sleepiness reaching 39 % [31] or hallucinations 60 % [36].

MSLT is an objective measure of daytime sleepiness. Patients receive the instruction not to fight against sleep. According to the AASM manual [4], the MSLT consists in five nap opportunities, scheduled at 2-h intervals, starting 2 h after awakening. Psychotropic drugs must be discontinued at least 2 weeks (depending on the half-life) before the recording. Each test is terminated after a 15-min sleep period or after 20 min if the patient does not fall asleep. Both sleep and REM sleep latencies are measured. Mean sleep latency below 8 min confirms the EDS. Latency below 5 min indicates severe sleepiness. Latency over 10 min is considered as normal. Between eight and ten, the interpretation depends on the clinical status of the patient. A sleep onset REM period (SOREM) defined as the occurrence of REM sleep within 15 min after sleep onset is also screened, with a narcoleptic phenotype defined as the presence of at least two SOREM on the MSLT.

As the sensitivity of MSLT to identify PD patients with disabling sleepiness remains questionable, further studies are required to validate the best methods to screen and objective hypersomnia in PD. Alternative methods to assess daytime sleepiness should be monitored in future in PD that may include the maintenance of wakefulness test or even more a prolonged 24-h continuous sleep recording in standardized conditions.

MWT is an objective measure of daytime wakefulness. It measures the ability to remain awake.

MWT consists in four 40 min tests, scheduled at 2-h intervals, starting 2 h after awakening. During the MWT the room is dimly illuminated and the subject lies in a semi-reclining position with the instruction to resist sleep [35, 36]. Subjects are not allowed to maintain wakefulness by using extraordinary measures such as slapping the face or singing. Recordings are monitored by a trained technologist. Each trial is terminated at the first onset of sleep (sleep onset) or, if sleep onset is not achieved, after a maximum in-bed duration of 40 min. Mean sleep latency below 11 min confirms the sleepiness [35, 36].

The MWT has clinical usefulness in evaluating response to treatment of sleepiness. In PD, a significant, but small, relationship between subjective (ESS) and objective (MWT) measures of daytime alertness hasbeen observed [37]. Interestingly, using MWT, Bliwise et al. have shown that relative to unmediated patients, all classes of dopaminergic medications were associated with reduced daytime alertness, and this effect was not mediated by disease duration or disease severity. Further, the results showed that increasing dosages of dopamine agonists were associated with less daytime alertness, whereas higher levels of levodopa were associated with higher levels of alertness [37].

PSG recording of wake and sleep activities at home or in the sleep laboratory is another way to explore sleepiness during the day in patients with PD. It allows observing sleep attacks and sleeping episodes during the day in naturalistic conditions. Only one study to our knowledge has evaluated sleep and wake cycle in PD using this method [38]. This study allowed them to show that sleep attacks are an extreme manifestation of increased daytime sleepiness.

Actigraphy is a noninvasive method for monitoring human rest–activity cycles. A small accelerometer unit is worn on the wrist by the patient to measure motor activity. Absence of movement, measured by actigraphy, has proved to be a good proxy of nighttime sleep quality and correlates with PSG and subjective sleep measures [39]. It has recently been shown that actigraphy correlates to sleep measures by PSG even in patients with PD where akinesia could have been misleading [40].

Objective measures to explore sleep–wake cycle and sleep disorders in PD encompass videopolysomnography, suggested immobilization test, and MSLT. Other methods such as MWT, 24-h ambulatory PSG, and actigraphy have also been recently used, mainly to explore sleepiness. Even if these methods are not specific to PD, they allow good identification of most sleep disorders. However, limits such as the absence of specific criteria for sleep stages identification in a disease where EEG is frequently impaired because of neurodegeneration and the low sensibility of MSLT to objectively measure sleepiness suggest that more accurate methods should be developed for the assessment of sleep and alertness in PD.