Table 1.

Definitions of sleep-wake and circadian disorders.

Phenomena	Description	Ref.
Sleep–wake disorders (SWD)/sleep disorders or disturbances	Generic term describing different disorders that may affect either night-time sleep, with difficulties falling asleep or maintaining sleep, or daytime, with difficulties staying awake, or a combination of both. These impairments manifest as frequent waking at night, excessive fatigue during the day, problems with sleep-wake schedule, nocturnal wandering and in general impaired function of sleep or sleep stages or partial arousals during sleep, potentially resulting in non-refreshing sleep. Causes of SWD are manifold, from sleep-related breathing disorders to neurological and psychiatric disorders.	[25,26,27,28,29]
Fatigue	Complaint of physical and/or mental exhaustion with difficulties in initiating or sustaining voluntary activities that are not significantly improved by increased rest or sleep. Subjective measurement by questionnaires such as the Fatigue Severity Scale (FSS), no objective measurement available.	[30,31]
Excessive Daytime Sleepiness (EDS)	Subjective difficulty maintaining wakefulness or alertness during major waking episodes of the day, with involuntary/unintentional and involuntary naps in monotonous situations, acquired need of scheduled napping during the day. Subjective measurement by questionnaires such as the Epworth Sleepiness Scale (ESS), objective measurement by multiple-sleep-latency test (MSLT).	[26,32,33]
Hypersomnia	An objective complaint of excessive sleep need. An objective assessment of an excessive quantity of sleep: at least 10 h of sleep duration over 24 h of the day with the nocturnal component providing at least 9 h of sleep duration. Objective measurement by actigraphy and/or polysomnography (PSG) ad libitum. More recent definitions of idiopathic hypersomnia adjusted the threshold to a sleep time >660 min in a 24-h cycle or a mean sleep latency in MSLT of ≤8 min.	[26,33,34]
Circadian rhythm disorders (CD)	Chronic or recurrent sleep disturbances due to alteration of the circadian system and/or misalignment between the environment and an individual’s sleep–wake cycle. Leads to advanced or delayed sleep phases up to day/night sleep inversion, interfering with social and work life requirements.	[35,36]
Insomnia	Difficulties initiating or maintaining sleep, even under appropriate circumstances and opportunity to sleep and without known cause of sleep disturbance (e.g., sleep-related breathing disorder, restless leg syndrome etc.). Furthermore, daytime consequences with impairment of social life or work are requirements for the diagnosis of insomnia. Subjective measurement by questionnaires such as the Insomnia Severity Scale (ISS).	[26,34]
Sleepiness/Somnolence	Describes a phenomenon that can be a symptom of medical, psychiatric, neurological or primary sleep disorders as well as a normal physiological state which is observed over a 24-h period in humans. It is most often defined as the tendency of an individual to fall asleep, which is also referred to as sleep propensity. Alternatively, it has been stated that sleepiness can be defined as an abnormal behavior when it either occurs at inappropriate time or when it is not desired.	[30]
Restlessness	Restlessness has been historically difficult to describe and has lacked a clear definition. Some literature defines restlessness as a state of aimless and poorly organized motor activity that stems from physical or mental unease. Other research papers use the term restlessness to measure sleep quality by asking subjects whether they suffer from disturbed sleep/insomnia. With respect to restlessness during sleep, this has previously been defined to be characterized by persistent or recurrent movements of the body, the presence of arousals and being briefly awakened during sleep.	[37,38,39]