Jennifer Vendetti& Joshua Rodgers IPHY 3700ARGDecember 6, 2005To Sleep or Not to Sleep?Whether it is more detrimental to awaken duringthe biological night or biological day.ORAre There Bad Times To Wake Up?An argument for the scientific community on the influence of circadian phase onsleep inertiaJoshua Rodgers and Jennifer VendettiFall 2005A paper for submission to SleepIntroduction In today’s 24-hour society, the luxury of regular sleep wake schedules can not be afforded by many professionals. Workers on rotating shift schedules may have to change the time that they sleep and wake up on a daily basis, without time for their bodies to appropriately adapt to the change. Also, firefighters, paramedics and physicians work long, irregular shifts and are regularly awakened from sleep at different biological times to respond to emergencies. The influence of biological time on the process of waking up and cognitive performance during this time are thus a major concern to these professionals as well as to the safety of those they are trying to help. Relative fluctuations in cognitiveperformance for an individual are largelyregulated by three biological processes: 1)how long one has been awake (i.e., thehomeostatic drive for sleep), 2) thebiological time that one is awake (i.e., biological night or biological day, referred to as 1circadian phase 0o and 180o, respectively), and 3) sleep inertia (impaired performance upon awakening from sleep) [1-6]. Each of these processes is defined further in the subsequent background section. The direct influence of these three processes individually on cognitive performance has received a great deal of attention and is well understood. However, the effects of the interactions of these processes have received lessattention, and many questions remain. Are there biological times when the process of waking up impedes cognitive performance to a greater extent? The key to answering this question is knowledge of the influence, if any, that circadian phase may play on the detrimental effects of sleep inertia. This article presents a review of the current state of knowledge on the influence ofcircadian phase on sleep inertia. The intent of this article is to highlight convincing data to support the hypothesis that the negative effects of sleep inertia are worse when one is awakened at the circadian trough (phase 0o) versus circadian peak (phase 180o) of core body temperature. Background The following paragraphs briefly define and describe the three aforementioned processes. The depth of explanation is kept to only that which is necessary for an understanding of the studies discussed later in this article. The human brain exhibits a homeostatic drive for sleep. This drive is manifested by a gradual, linear increase of sleep propensity, beginning upon awakening, increasing throughout the day, and exponentially dissipating during sleep [3]. It is believed that this drive is the result of adenosine buildup in certain brain regions. Extended periods 2without sleep (i.e. sleep deprivation) thus result in an increasing subjective sense of sleepiness and decrements in objective measures of cognitive performance [17]. The “master biological clock” is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, and, via cycles in the activity of specific genes, creates a sinusoidal, near 24-hour rhythm regulating numerous biological functions throughout the body. This rhythm is referred to as the Circadian Rhythm (circa- meaning near and -dian meaning day) [3]. The circadian rhythm of core body temperature (CR-CBT) progressively promotes alertness throughout the day, peaks in the evening hours, and then gradually diminishes, promoting sleep at night. This process works opposite that of the homeostatic drive for sleep to create a fairly stable level of alertness throughout the day (Figure 1). Additionally, the SCN receives time cues, such as light intensity and meals, inorder to properly keep time [2]. Sleep inertia is the state of lowered arousal occurring immediately after awakening from sleep and producing a temporary decrement in subsequent performance as the brain transitions from a sleep state toward full alertness [7, 8]. The effect of sleep inertia on performance may depend in part on the task being performed. For example, it has been reported that accuracy [9] was less affected than cognitive speed [10]. Sleep inertia impairs working memory, reaction time, math performance, decision making, subjective sleepiness, visual search, logical reasoning and grip strength [7]. Factors contributing to the general severity of sleep inertia include time since awakening [8-10], prior drug use [11], and stage of sleep prior to awakening. In general, awakening from deep sleep (slow wave sleep) results in more impairment than awakening from REM and stage 2 sleep except when REM and stage 2 are recently preceded by deep sleep [8, 10]. 3Depending on these factors, sleep inertia can have measurable deleterious effects on performance for more than 2 h after awakening [9]. It is also important to note, especially for emergency crews and physicians, that sleep inertia significantly impairs performance even when a person is exposed to high stress events upon awakening [8].Research on the influence of circadian phase on sleep inertiaThe influence of circadian phase on the deleterious effects of sleep inertia on cognitive performance is unresolved (Table 1). Over the last two decades, three laboratory studies have directly attempted to quantify the effect of circadian phase on sleep inertia in healthy young adults (ages 18-35). Of the three, two studies [12, 13], conducted in 1976 and 1993, found no relationship between circadian rhythm and sleep inertia, and concluded that the length and timing of a bout of sleep deprivation were moreimportant considerations than circadian placement of a nap. However, in 2005, Rodgers, used a more sophisticated research protocol and found the degree of sleep inertia when awakening at the CR-CBT trough (phase 0o) to be significantly worse than that at the CR-CBT peak (phase 180o). Additionally, several studies [10, 15,16,17] indirectly support this idea by providing data on the core body temperature effects on sleep inertia and the influence of CR-CBT on performance throughout the day or at various nighttime awakenings. Prior research The two early studies attempted to determine the effects, if any,