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Wondering what circadian rhythms are? Or how night terrors and nightmares differ? Discover our Sleep Dictionary – your A-Z guide to all things sleep-related. If you’re confused about the jargon in the world of sleep, read our sleep glossary.

Bedding items, including comforters, pillows and mattress protectors/toppers, that are designed to repel allergens, reducing or even eliminating night-time allergy symptoms for sufferers. For more, see Hypoallergenic Bedding below.

What is biphasic sleep? It’s a routine which involves going to sleep twice each day. The siesta, in which people take a nap during the hottest part of the day, is a good example of a biphasic sleep cycle – although the two sleeping periods can be taken at any time during a 24-hour period.

Light emitted by electronic devices such as smartphones, computers and tablets. This light can send signals to the brain that it is still daylight, causing a person to feel more alert rather than sleepy. This is why it’s a good idea to stop using electronic devices at least an hour before bed.

The daily cycle your body goes through including sleeping, waking and eating. Circadian rhythms are regulated by the hypothalamus and can be affected by external factors such as:

  • Sunlight
  • Temperature
  • Blue light emitted by electronic devices

Also known as Stage 4 sleep, deep sleep is the phase during which the body regenerates and repairs itself. It typically lasts around half an hour during each sleep cycle. Spending enough time in this phase of sleep is important to help you feel refreshed when you wake.

To desorb is to release moisture into the atmosphere. In relation to sleep, it refers to the process by which certain bedding materials can help to keep you cool and dry at night, by absorbing the moisture from sweat and then desorbing it.

Natural fibers tend to be more effective at this process than synthetics. Wool is scientifically proven to absorb significantly higher volumes of moisture when compared with feather/down and polyester.

What is down material? Down are the fine under-feathers found on ducks and geese. They are lighter and fluffier than the outer feathers and are there to keep the bird warm by providing extra insulation. This insulating property is why down is sometimes used as a filling for comforters and pillows.

However, while an effective insulator, down is not as breathable as some other natural fibers and can’t manage moisture effectively.

Allergic reaction to the proteins found in dust mite droppings, causing symptoms such as itching, coughing or a runny nose. Symptoms are usually experienced at night because dust mites thrive in warm, humid environments such as beds, bedding and carpets. Because sufferers experience the symptoms mainly or only at night, many people are not aware that they have a dust mite allergy.

The best way to tackle dust mite allergy is to create a sleeping environment in which dust mites can’t thrive. This means choosing wool bedding and mattresses over synthetic or feather/down alternatives, as well as natural wool carpets, rugs and curtains.

Comforters, pillows and other bedding products filled with a combination of down (soft under-feathers) and the tougher outer feathers of birds – usually ducks or geese. While feather/down is a natural filling, it is not hypoallergenic, nor can it be entirely ethical.

The region of the brain responsible for regulating the body’s circadian rhythms, affecting both sleepiness and alertness. Problems with the hypothalamus can be connected with sleep conditions such as insomnia and narcolepsy.

Bedding, including comforters, pillows and mattress toppers, that are designed to create an environment in which allergens find it difficult to survive. Key allergens typically found within non-hypoallergenic bedding include dust mite droppings and fungal spores.

To get maximum anti-allergy benefit, some bedding materials are more effective than others. Wool is very effective at repelling allergens naturally. Look out for the Allergy UK symbol on approved wool-filled hypoallergenic bedding

If you’re having difficulty getting enough sleep over a sustained period of time, it could be that you’re suffering from insomnia. What is insomnia? Common signs of insomnia include disturbed sleep during the night and difficulty drifting off to sleep. Episodes of insomnia can be as short as a few days or can last months or longer.

Next in our sleep dictionary is loft, which refers to the fluffiness of a comforters or pillow. Traditionally used in relation to feather/down products, it can also be used to describe wool-filled comforters.

How does melatonin affect sleep? It’s a hormone that the body produces naturally to help a person sleep. If you use any blue light emitting devices, including your laptop or cellphone, these devices can suppress the level of melatonin that your body produces. That means your body won’t think it’s time for sleep, and could explain why you’re struggling to sleep.

What is a monophasic sleep pattern? It’s a daily routine during which a person sleeps once every 24 hours. This is the familiar sleep cycle that most adults follow.

What is sleep narcolepsy? It’s a condition that causes sudden sleeping episodes, usually during the day. A person with narcolepsy can fall asleep without warning at any time and place.

What can cause narcolepsy? It’s often caused by chemicals that regulate sleepiness and wakefulness reacting in the brain. However other causes are unknown. Narcolepsy is a rare condition and although it can affect people of any age, it is more common in adults over the age of 20.

Allergies can get worse at night and make it difficult for you to get the best night’s sleep. Find out more about what’s causing your night time allergies.

Unpleasant dreams that can cause a person to wake during the night. They are more common in young children than older children or adults, although they can affect people at any age.

What’s the difference between nightmares and night terrors?

Not the same as nightmares; night terrors are characterized by shouting or screaming during sleep and moving in an agitated manner. By contrast to nightmares, a person who has experienced night terrors usually can’t remember the incident once they are awake. Although night terrors can affect people of all ages, they are more common in younger children.

Overheating while asleep; usually resulting in excessive sweating at night. Night sweats can affect people of all ages. But they are more common for:

  • Women experiencing the menopause
  • People with specific medical conditions
  • Young children and babies as they are unable to regulate their own body temperature


What is polyphasic sleep? It’s a routine in which a person sleeps three or more times during a 24-hour period. Sleep can be divided into naps of equal duration or divided up unevenly. Young babies tend to follow a polyphasic sleep routine before developing a biphasic and eventually monophasic sleep cycle as they get older.

A power nap is a short doze, usually taken during the daytime, with the aim of feeling refreshed and more focused. Read more about the pros and cons of a power nap.

Also known as Stage 5 Sleep, REM sleep. This is the phase during which you experience dreams. You usually enter this stage of sleep around 90 minutes after falling asleep.

It’s a neurotransmitter that sends signals to your nerve cells. Like melatonin, it’s a hormone produced by the body to help a person relax and feel sleepy.

What is sleep apnoea and what causes it? It’s a condition in which the throat narrows during sleep, restricting breathing and causes a person to wake. Signs of sleep apnoea can include:

  • Frequent waking at night
  • Snoring
  • Fatigue

Although sleep apnoea can affect people of any age, it is most common in men over the age of 30.


The five stages the body goes through during sleep, from light sleep through to deep sleep, before entering REM sleep. The average person completes this cycle several times each night. Find out more about the different stages of sleep.

It’s so important to try and create the best sleep environment to get a good night’s sleep every night. Some external factors you could change include:

  • Temperature
  • Light
  • Noise
  • Scent
  • Any allergens or chemicals in your bedroom

The best sleep environment needs to be healthy for you to sleep well. That means you’ll need to make sure you’ve got the right bedding and room design.


Good sleep hygiene can improve the quality of your sleep. There are things you can do to make sure you have good sleep hygiene, making sure you sleep well every night, for example:

  • Setting a regular bedtime routine
  • Regulating what you eat before bed
  • Choose the right environment you choose to sleep in

Learn how you can sleep better by evaluating your sleep hygiene.


What is sleep inertia? It’s a feeling of drowsiness experienced when you first wake up. While most people experience sleep inertia, this usually lasts for no more than half an hour. Experiencing sleep inertia for longer periods can be a sign of poor quality sleep.

Waking up feeling refreshed does not just depend on the duration of sleep; sleep quality also matters. The amount of time spent in regenerative, deep sleep can enhance overall quality of sleep.

Sleep paralysis is the temporary inability to move or speak upon waking or falling asleep. It can affect some people just once or can occur more regularly, but is not usually a sign of an underlying health problem. Sleep paralysis tends to happen during the rapid eye movement (REM) stage of the sleep cycle, which is the deepest and longest cycle where dreams are most likely to occur.

People who experience sleep paralysis may find that they are aware of their surroundings, but feel unable to move or talk. An episode should only last a few seconds but it can disrupt your sleep. If sleep paralysis is regularly affecting your sleep, talk to your physician about your options. They may be able to refer you to a sleep specialist.

A common sleeping condition where the soft palate in the mouth, nose or throat vibrates to make a noise while people are sleeping. Snoring differs from person to person, and can range in volume and how regularly it happens.

Snoring can sometimes indicate other issues, such as sleep apnoea, so is important to monitor if it becomes more frequent. Visit your doctor for more support if you find that snoring is affecting your sleep and how alert you are during the day.

Also known as sleepwalking – when a person walks or carries out activities while remaining in a state of sleep. Somnambulism covers everything from brief episodes of sleep walking, through to more extended incidents where a person may carry out everyday tasks such as getting dressed or even leaving the house.

Sleepwalking or somnambulism causes could include:

  • Stress
  • Fever
  • Some types of medication

Although sleepwalking can affect people of all ages, it is more common in children. But it’s nothing to worry about, as most children will grow out of it.


Somniloquy, most commonly known as sleep talking, is a sleep disorder where the sleeper speaks in their sleep yet is unaware of doing so. From mumbles and gibberish to full sentences and conversations, the range of somniloquy can vary from person to person. Sleep talking is not harmful, but may cause embarrassment if heard by a partner or nearby sleeper.

Comforters, pillows and mattress toppers/protectors filled with synthetic fibers, usually polyester. While synthetic bedding tends to be cheaper than natural alternatives, it does not provide the same heat-regulating and anti-allergy properties that can enhance quality of sleep.

What is a tog rating system? It’s a method of measuring the warmth of a comforter. A tog measures thermal resistance, with lower numbers indicating cooler comforters and higher numbers indicating a warm comforter. The rating system was developed in the 1940s to measure the warmth of synthetic fibers and is today used mainly for comforters.

Because the tog rating system was devised to measure synthetic fabrics, it is not ideally suited to measuring the warmth of intelligent natural fibers such as wool. That’s why when looking at wool comforters, you will often see a tog range (e.g. 3-6 tog) rather than the set tog rating you will find on polyester and feather/down comforters.

This is because wool has temperature-regulating properties to keep a person warm when they’re cool, but cool when they’re hot, making it difficult to determine a single tog rating. By contrast, because synthetic comforters do not regulate temperature in the same way, a single tog rating is possible.

White noise refers to the sound heard when a combination of frequencies is emitted at a similar level. Much like the fuzzy frequencies you hear before finding an FM radio channel, white noise is a low level mixture of tones that many people find soothing.

That’s why white noise machines and apps are now popular techniques that many use to help them fall asleep. The frequencies blend with background noise, allowing you a more peaceful, undisrupted sleep. Some people don’t like the noise that white noise makes so opt for the sound of rainfall or music instead. However, music doesn’t quite have the same effect as it does not contain the wide range of frequencies required to mask all background noises in your bedroom.

Wool-filled bedding, including comforters, pillows and mattress protectors and toppers. Wool is used to fill the inside of these products, harnessing the anti-allergy and heat-regulating benefits of this natural fiber, while the outside is usually encased in soft cotton.

Still want to know more after reading our sleep dictionary? Learn more about sleep – and how to sleep better – with our Sleep Health and Advice Hub.