We've all been there: you leave the supermarket after doing your weekly shop and take a minute or two to remember where you parked the car. Or you go over the same passage of a book several times, yet absorb nothing of what you've read.
These momentary lapses in memory or concentration are normal for most people. We all get a little bit hazy of mind when we're tired, overwhelmed or stressed.
But sometimes, these blips happen often and disrupt life. Periods of 'brain fog' can come like huge waves and render you unable to think clearly for hours or days, incapable of performing everyday tasks or holding conversations.
Defining brain fog
Brain fog is a general term for a set of symptoms affecting the cognitive processes. It isn't a medical condition in itself, but rather occurs as common feature of other conditions.
They include autoimmune disorders like lupus, coeliac disease and multiple sclerosis; neurological conditions such as myalgic encephalomyelitis (ME) and chronic migraine; depression and anxiety; the systemic pain syndrome fibromyalgia; and following treatment for cancer.
When brain fog strikes, a person can have trouble remembering things and processing information. They might also find it difficult to concentrate and pay attention, or that their thoughts are disorganised.
Although brain fog is neither progressive nor associated with declining intellect, it can be completely bewildering and erode a person's confidence and self-esteem.
Some people with brain fog experience problems with speech and language; it can be hard putting their thoughts into words and finding the right words, and their speech might be slow and confused.
'Difficult to accept'
For 40-year-old Joe Nutkins, it's these language problems that are the most troubling feature of brain fog. "I've always loved using wordplay, both spoken and written, but the past few years I find it hard to think of the word I'm trying to say or write - even simple, everyday words," she says.
"Spelling can be very difficult, too. It makes me feel like I've regressed in my brain function and my vocabulary is dwindling. It's the most upsetting aspect of brain fog, and the most difficult to accept."
Joe, who lives in Tendring, Essex, experiences brain fog as a symptom of both fibromyalgia and ME. It affects her to varying degrees every day, but is often debilitating. She explains that it sometimes presents as a mental block that stops her from thinking clearly.
"It can sometimes take several hours just to reply to a few messages, compared to when I'm clearer of mind when I can do the same work in less than an hour," she comments.
Other times, Joe says, brain fog affects her short-term memory. She regularly forgets her dogs' names and has to consciously recall how to do the simplest of tasks - starting up her van or pulling up the handbrake after parking, for example.
Brain fog affects sufferers to varying degrees and with varying frequency, but people commonly report that it's the most debilitating part of their illness.
In an online survey of people with ME carried out by the ME Association, one third (33%) of respondents said their brain fog is constant and moderately or very disabling. An additional 23% said their brain fog varies between moderate and severe.
Dr Charles Shepherd, medical adviser to the ME Association, explains that brain fog - or cognitive dysfunction, as he terms it - is often worsened by mental exertion.
"In the same way that too much physical activity will quickly produce muscle fatigue, prolonged or intense mental activity will bring on or exacerbate brain fatigue and the cognitive problems that accompany it," he comments.
Similarly, the severity of cognitive dysfunction can be associated with a person's general state of health.
"Other medical problems, such as frequent headaches, pain, sleep disturbance, anxiety and depression, can all cause cognitive dysfunction - regardless of whether or not you have ME," Dr Shepherd says. "So, if any of these are present, they are likely to be exacerbating the problem."
A steady pace
For this reason, many people find the best way of coping with brain fog is by 'pacing' - balancing activity with rest to avoid becoming overwhelmed.
"Pacing mental activities is a key aspect of management," says Dr Shepherd. "Overall, this means finding a comfortable baseline of mental activity and splitting it up into small, manageable chunks with rest or relaxation periods in between."
He adds that it's important to stop any cognitively demanding tasks or activities before you get mentally tired, and to avoid pushing yourself beyond your limitations once you've established what they are.
There are a number of practical steps you can take to minimise the impact of brain fog. If you experience short-term memory loss, it might be helpful to keep lists of important activities or tasks, appointments and things you need to do each day - and to refer to them often.
Having 'homes' for items you might often misplace, like glasses, keys and medication, can help, as can focusing on one activity at a time rather than attempting to multi-task.
If processing information is a problem for you, try repeating any new information back to yourself - this can make it easier to retain.
Most essential is ensuring you have the support you need. If you find it difficult to cope with the effects of brain fog, it's worth talking to your GP and exploring psychological remedies, says Dr Shepherd.
"In addition, it's important to make family, friends and work colleagues aware of the difficulties you're having and explain how they can help you cope."