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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.

Anti-allergy bedding
Bedding items, including duvets, 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.

Biphasic sleep
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.

Blue light
Light emitted by electronic devices such as smartphones, computers and tablets. This light is capable of sending 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.

Circadian rhythms
The daily cycle your body goes through including sleeping, waking and eating. Circadian rhythms are regulated by the hypothalamus (see below) and can be affected by external factors such as sunlight, temperature and also the blue light emitted by electronic devices.

Deep sleep
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 fibres tend to be more effective at this process than synthetics; wool in particular is scientifically proven to absorb significantly higher volumes of moisture when compared with feather/down and polyester.

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 duvets and pillows. However, while an effective insulator, down is not as breathable as some other natural fibres and cannot manage moisture effectively.

Dust mite allergy
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, due to the fact that 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.

Feather/down bedding
Duvets, 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.

Hypothalamus 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.

Hypoallergenic bedding
Bedding, including duvets, 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, in particular, is highly effective at repelling allergens naturally. Look out for the Allergy UK symbol on approved wool-filled hypoallergenic bedding.

Difficulty getting enough sleep over a sustained period of time. 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. Loft
Next in our sleep dictionary is loft, which refers to the fluffiness of a duvet or pillow. Traditionally used in relation to feather/down products, it can also be used to describe wool-filled duvets.

A hormone produced naturally by the body to help a person sleep. This hormone can be suppressed by blue light emitting devices.

Monophasic sleep
A daily routine during which a person sleeps once every 24 hours. This is the familiar sleep cycle that most adults follow.

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. The condition can be caused by a lack of certain chemicals in the brain that help to regulate sleepiness and wakefulness, 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.

Night allergies
Worsening or onset of allergy symptoms at night. See dust mite allergy above.

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. Nightmares that are characterised by screaming, shouting or lashing out during sleep are more likely to be Night Terrors, particularly if the person can’t remember the episode afterwards (see Night Terrors below).

Night sweats
Overheating while asleep; usually resulting in excessive sweating at night. Night sweats can affect people of all ages although they are more common in women experiencing the menopause, people with specific medical conditions, and young children and babies as they are unable to regulate their own body temperature.

Night terrors
Not the same as nightmares; night terrors are characterised 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.

Polyphasic sleep
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.

Power nap
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.

REM sleep
Also known as Stage 5 Sleep, REM sleep. This is the phase during which you experience dreams. It is usually reached around 90 minutes after falling asleep.

Like melatonin, serotonin is a hormone produced by the body to help a person relax and feel sleepy.

Sleep apnoea
A condition in which the throat narrows during sleep, restricting breathing and causing a person to wake. Signs of sleep apnoea can include frequent waking at night, snoring and fatigue. Although sleep apnoea can affect people of any age, it is most common in men over the age of 30.

Sleep cycle
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.

Sleep environment
The external factors within the physical environment that can impact a person’s ability to sleep. These include temperature, light, noise, scent, and also other elements such as the presence of allergens or chemicals in the atmosphere. Creating a healthy sleep environment involves careful selection of the right bedding, beds and mattresses, as well as room design.

Sleep hygiene
Steps to follow to get a better night’s sleep. Good sleep hygiene can cover anything from setting a regular bedtime routine, to regulating what you eat before bed, and even the environment you choose to sleep in.

Sleep inertia
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.

Sleep quality
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
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 GP 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 GP 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 can be triggered by factors including stress, fever, and even 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.

Synthetic bedding
Duvets, pillows and mattress toppers/protectors filled with synthetic fibres, 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.

Tog rating system
A method of measuring the warmth of a duvet. A tog measures thermal resistance, with lower numbers indicating cooler duvets and higher numbers indicating a warm duvet. The rating system was developed in the 1940s to measure the warmth of synthetic fibres and is today used mainly for duvets.

Because the tog rating system was devised to measure synthetic fabrics, it is not ideally suited to measuring the warmth of intelligent natural fibres such as wool. That’s why when looking at wool duvets, 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 duvets. 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 duvets do not regulate temperature in the same way, a single tog rating is possible.

White Noise
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 bedding
Wool-filled bedding, including duvets, 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 fibre, while the outside is usually encased in soft cotton.

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