Tracking Sleep Stages In Rats: Is It Possible?

Sleep is a complex and multifaceted process that has been a subject of extensive research in humans and animals alike. Rats, in particular, are the most frequently used species in animal sleep studies. While human sleep stages have been clearly defined and are well understood, the same cannot be said for rats and other rodents. This is partly due to the fact that rats are polyphasic sleepers, meaning they experience multiple sleep episodes throughout the day, in contrast to the typical monophasic sleep pattern observed in humans. This creates challenges in defining when one sleep episode ends and another begins. However, advancements in technology and data-driven approaches have led to the development of automatic sleep-staging methods, such as the rule-based five-stage scoring system, which can accurately classify vigilance states into wake, NREM, and REM sleep. These tools provide valuable insights into the complex world of rat sleep and contribute to our understanding of sleep architecture.

Sleep-wake cycle differences between rats and humans

Rats are polyphasic sleepers, meaning they sleep multiple times in a 24-hour period. They cycle through vigilance states of brief wake (BW), slow-wave sleep (SWS), and rapid-eye movement sleep (REMS). In contrast, humans are typically monophasic sleepers, usually sleeping once for around 8 hours per day.

The sleep-wake cycle of 20 male albino rats was observed over a 12-hour period from 8:00 a.m. to 8:00 p.m. The rats were found to be awake 24% of the time, in SWS 67% of the time, in paradoxical sleep (PS) 9% of the time, and asleep a total of 76% of the time. They experienced a mean of 396 wake-ups from either SWS or PS per day.

The sleep-wake cycle of rats is influenced by the illumination cycle, with differences observed between the light and dark periods. During the light period, rats exhibit brief periods of wakefulness interspersed with SWS and REMS. In the dark period, rats give the impression of being awake at the onset of PS, with open eyes and body posture indicating quiet wakefulness, despite exhibiting EEG waves associated with SWS.

The sleep-wake cycle in rats can be studied through automatic sleep classification systems that record and analyze bioelectrical signals such as EEG, EMG, and movement indicators. These systems have been used to develop a 5-stage or 6-stage classification of sleep patterns in rats, including active and quiet waking, quiet and deep sleep, pre-REM, and REM sleep.

In summary, rats and humans differ in their sleep-wake cycles, with rats being polyphasic sleepers and humans typically being monophasic sleepers. The sleep patterns of rats are influenced by illumination cycles and exhibit unique characteristics such as brief wakefulness and the absence of REMS episodes between vigilance states.

Polysomnographic measures of sleepiness

Polysomnography (PSG) is a multi-parameter sleep study and a diagnostic tool in sleep medicine. It is performed overnight while the patient is continuously monitored by a technologist. PSG monitors many body functions, including brain activity (EEG), eye movements (EOG), muscle activity or skeletal muscle activation (EMG), and heart rhythm (ECG).

A polysomnogram will typically record a minimum of 12 channels, requiring a minimum of 22 wire attachments to the patient. These channels vary in every lab and may be adapted to meet the doctor's requests. A minimum of three channels are used for the EEG, one or two measure airflow, one or two are for chin muscle tone, one or more for leg movements, two for eye movements (EOG), one or two for heart rate and rhythm, one for oxygen saturation, and one each for the belts, which measure chest wall movement and upper abdominal wall movement.

Polysomnography is used to diagnose or rule out many types of sleep disorders, including narcolepsy, idiopathic hypersomnia, periodic limb movement disorder (PLMD), REM sleep behavior disorder, parasomnias, and sleep apnea. It can also be used to screen for drug effects on sleep. The test can be used to evaluate sleepiness by measuring the patient's sleep cycles, which are rapid eye movement (REM) sleep, and non-rapid eye movement (non-REM) sleep. Observing the sleep cycles, along with the body's reactions to the changes in these cycles, can help identify disruptions in sleep patterns.

In rats, an automatic sleep classification system can be used to record and analyze bioelectrical signals over extended periods. The analysis is based on three signals: the parieto-occipital EEG, nuchal EMG, and a movement indicator signal. The automatic sleep staging into six stages (active and quiet waking; quiet, deep, pre-REM, and REM sleep) is performed offline.

Vigilance cycling

Rats are polyphasic sleepers, meaning they have multiple sleep episodes in a 24-hour period. However, there is no formal definition of when one sleep episode ends and another begins.

A study examining the distribution of wake episode durations found that if a wake episode exceeds 300 seconds, it is considered long-duration wake (LDW), and if it is less than 300 seconds, it is considered brief wake (BW). This separation is supported by an analysis of the EEG power spectrum in BW versus LDW.

The sleep episodes between LDW periods are termed vigilance cycling (VC) because the rat is cycling between the vigilance states of BW, slow-wave sleep (SWS), and rapid-eye movement sleep (REMS). VC characteristics differ between the light and dark periods, with SWS pressure decreasing and REMS pressure increasing as the VC period continues.

The concept of VC provides insights into the description of sleep patterns in rats and may be important for developing a complete understanding of sleep behavior in this animal. VC episodes appear to be regulated and meaningful to the sleep behavior of rats.

Furthermore, the sleep-wake cycle in rats differs from that of humans, who are typically monophasic sleepers with one ~8-hour sleep session per day. The sleep cycle in humans is often measured as the time from one REMS episode to another (REMS-REMS cycle), but rats frequently cycle through SWS to wake without REMS episodes. This limits the usefulness of the REMS-REMS cycle as a parameter for sleep in rats.

To overcome this challenge, researchers have developed a rule-based automatic five-stage scoring method for rats, which includes wake/NREM1/NREM2/transition sleep/REM stages. This method has shown high accuracy and reliability in classifying sleep patterns in rats.

Sleep scoring

The sleep cycle in rats can be broadly divided into wake, non-rapid eye movement (NREM), and rapid eye movement (REM) sleep. However, further distinctions can be made within these categories. For example, wakefulness can be divided into active and quiet waking, while NREM sleep can be subclassified into light and deep sleep.

A typical sleep scoring protocol for rats might look like this: First, a threshold for electromyogram (EMG) activity or movement is set to distinguish wake from sleep. Then, the sleep periods are divided into NREM and REM sleep by analyzing the theta/delta spectral power ratio and setting a threshold. An expert scorer will then visually inspect the data and correct any misclassifications, which most commonly occur during transitions between sleep stages.

Several automated sleep scoring systems have been developed to streamline the process of sleep scoring in rats. These systems use algorithms to analyze electroencephalogram (EEG) and EMG data and classify sleep stages with high accuracy. One such system, described by Wei et al. (2019), achieved 92.32% agreement in five-stage sleep scoring and 94.97% agreement in three-stage sleep scoring when compared to manual scoring by human experts.

Sleep stages

Sleep is an important area of research, with sleep classification occupying a significant role in medical sleep disorder research, basic neuroscience research, and industrial and transportation safety. Rats are the most frequently used species in animal sleep studies.

Rats are polyphasic sleepers, meaning they have multiple sleep episodes in a 24-hour period. A typical sleep cycle in rats involves cycling through vigilance states of brief wake (BW), slow-wave sleep (SWS), and rapid-eye movement sleep (REMS). However, there is no universally agreed-upon standard for sleep stages in rats, and the field of sleep research in rodents is still evolving.

The sleep cycle in rats can be studied through various methods, including manual and automatic approaches. Manual sleep classification in rats has traditionally been a time-intensive task, involving the use of electroencephalogram (EEG) and electromyogram (EMG) recordings. The spectrum of rodent sleep is typically divided into three categories: awake, rapid-eye-movement (REM) sleep, and non-rapid-eye-movement (NREM) sleep. These stages can be distinguished by EMG power and EEG power in particular frequency bands.

Recent advancements in technology have led to the development of automatic sleep classification systems for rats. These systems record and analyze bioelectrical signals such as EEG and EMG data over extended periods. Some automatic scoring algorithms can classify vigilance states into 3-stage or 5-stage wake-sleep patterns with high agreement with sleep experts.

Additionally, a novel 5-stage sleep-scoring method for rats has been proposed, which includes wake, NREM1, NREM2, transition sleep, and REM stages. This method has shown high accuracy and reliability in classifying sleep patterns in rats with acid-induced widespread hyperalgesia.

In summary, while there is no standardized manual for sleep scoring in rodents, advancements in computing power and machine learning offer promising avenues for developing more sophisticated automatic sleep classification systems for rats. These systems will enhance our understanding of sleep architecture and sleep behaviour in these animals.

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