We all experience the blues from time to time but how can you tell if it's perhaps something more?

Read on for tips on how to spot the signs in you - or a loved one - that could be something deeper...

A constant shadow

Because being depressed is not something that comes and goes according to how your day or week is panning out – it lingers, heavily, sometimes for months, sometimes for years.

[Read more: 11 easy ways to boost your mental health in 2016]

“Most of us will feel down from time to time – we all have good days and bad days,” says Beth Murphy, head of information at Mind, the mental health charity.

“However, if you’re feeling low for a couple of weeks or more without much change in mood, or such feelings return over and over again, this could be a sign of depression.”

Spreading the word

The trouble is, not enough people know this. In fact, a staggering 90% of people with mental health problems say that stigma and discrimination have a negative effect on their lives.

“We know some people hold the view that you can 'buck up' or believe in 'mind over matter' but this is not always helpful for those experiencing a mental health problem,” explains Murphy.

Mind, together with Rethink Mental Illness, runs Time to Change, an anti-stigma campaign in England and Wales, and since its launch in 2007, it has seen an 11% reduction in discrimination as reported by people with mental health problems.

“We are making progress,” Murphy says, but quickly admits “we still have a long way to go”.

How to know

Part of this long journey is of course awareness, reminding people there’s a crucial difference to ‘pulling yourself together’ and battling the symptoms of true, crippling depression.

These symptoms will naturally vary from person to person, but watch out - both in yourself and in those you love – for any of the following signs. If you are worried you (or they) might need help, see your GP, or contact Mind.

[Read more: 7 simple ways to support your child’s mental health]

1. Psychological symptoms

Clinical depression manifests itself in many ways, but many sufferers will experience a general feeling of “low spirits, restlessness and irritability” says Murphy.

“They often see no point in the future, feel numb and disconnected from other people and gain no pleasure from things they usually enjoy.”

Frankie Sikes, a psychotherapist, trainer and consultant of a mindfulness nature-walking retreat, adds that people who are depressed also often experience “feeling lethargic, drained and intimidated by even small tasks” and are “overly critical or down on themselves”.

2. Physical symptoms

Depression is usually classed as a mental illness, but it quickly spreads its hold onto a sufferer’s health.

Sikes notes “having unusual physical symptoms like stomach upsets, head aches and back aches” as well as struggling to focus, concentrate or make decisions.

Another common symptom, adds Murphy, is losing interest in sex, and avoiding all forms of intimacy. Many people afflicted by depression also notice a change in their sleep patterns, be that the need to sleep much more, or sleeping too little.

3. Social symptoms

Dreading work? Avoiding contact with friends? Neglecting hobbies? Again, while it’s natural to sometimes go through dips of interest in your job or social life, a sustained dread of socialising or coping with daily office tasks can be a sign of depression.

As well as avoiding aspects of daily life, “another main sign of depression is engaging compulsively in escapist activities like drug and alcohol use,” says Sikes, “or even excessive internet use.”