It's also estimated that one in six experience a neurotic disorder such as anxiety or depression.
Talking about the symptoms of anxiety disorders - specifically generalised anxiety disorder (GAD) - psychologist Douglas Mennin reveals that these disorders can affect different people in different ways and on varying scales.
But, he says, "they all provoke extreme fear or worry that interferes with daily life".
Symptoms of anxiety
- Chronic or unsubstantiated worry.
- Repeated, random panic attacks or persistent worry and anticipation of another panic attack.
- Irrational fear or avoidance of an object, place or situation that poses little or no threat or danger.
- Performing uncontrollable, repetitive acts such as washing your hands or checking things over and over.
- Ongoing and recurring nightmares, flashbacks and emotional numbing relating to an emotional event in your life.
Mennin adds that symptoms of anxiety may pass quickly, and in other people they may last for a long time.
Generalised anxiety disorder (GAD)
According to NHS Choices, GAD is a long-term condition that causes you to feel anxious about a wide range of situations and issues, rather than one specific event.
It is estimated to affect about one in every 25 people in the UK.
"People with GAD feel anxious most days and often struggle to remember the last time they felt relaxed," reads the site.
"It can cause both psychological (mental) and physical symptoms. These vary from person to person, but can include feeling restless or worried and having trouble concentrating or sleeping."
In the video above, Mennin reveals that women are twice as likely to be affected as men.
"People with this disorder experience constant worry, it disrupts social activities and interferes with work, school or family," he says.
"They might also experience physical symptoms such as muscle tension and fatigue, restlessness and irritability, or gastro and intestinal discomfort of some kind."