What Is SAD? Causes, Warning Signs & How to Be There for Someone

When someone you know seems different this winter, it could be Seasonal Affective Disorder. Here's how to spot the signs and support them through it.

Seasonal Affective Disorder (SAD) is a form of seasonal depression that typically appears during the autumn and winter months, when daylight hours are shorter. For this reason, it’s often called winter depression.

Around 1 in 15 people in the UK experience symptoms of SAD between September and April, so someone you know may be affected. The good news is that recognising the signs and knowing how to support them can make everyday life feel brighter throughout the year — even on the cloudiest days.

What is seasonal affective disorder (SAD)?

Seasonal Affective Disorder is a type of depression that follows a predictable seasonal pattern. Symptoms usually worsen in the darker months and improve when spring and summer bring more natural daylight.

When does SAD start and what are the worst months?

SAD usually starts in late autumn, around October or November, as daylight hours shrink. Symptoms tend to be at their most severe in January and February, before gradually easing as spring brings longer, brighter days.

What causes SAD?

SAD is linked to reduced sunlight exposure, which disrupts several natural processes in the body:

1. Overproduction of melatonin - Melatonin is the sleep hormone. In winter, some people produce too much of it, leading to increased tiredness and low energy.

2. Lower serotonin levels - This brain chemical affects mood, appetite and sleep, with links to feelings of depression. A drop in serotonin can therefore throw these out of kilter.

3. Changes in internal body clock - We rely on daylight to regulate our body clock. Shorter days can throw this rhythm off balance, contributing to SAD symptoms.

Risk factors and vitamin D

SAD can affect anyone, but it’s diagnosed more often in women, younger adults, and people with a family history of depression, and it becomes more common the further you live from the equator, where winter daylight hours are shortest. Low vitamin D — which the body makes mainly from sunlight — has also been linked to low mood, so it’s worth reading our guide to winter vitamin supplements if you think this could be a factor for you.

Winter vs summer-pattern SAD

Most people picture SAD as a winter illness, but a small number of people experience a summer pattern instead, with symptoms like insomnia, poor appetite, agitation and weight loss appearing in the warmer months. The signs below reflect the more common winter pattern — if what you’re noticing doesn’t quite match, it’s still worth describing the pattern to a GP.

Signs and symptoms of SAD

It’s normal to feel a little sluggish during winter, but if someone you know is acting out of character, they may be experiencing SAD. For around 3% of people, symptoms can significantly impact daily life.

Look out for:

Persistent low mood - They may appear irritable, anxious or withdrawn. Conversations feel strained, and they might seem quieter or snappier than usual.
Avoiding social situations -Someone who normally enjoys going out may decline invitations or prefer staying indoors.
Heightened emotions -They may cry more easily, show frustration or use negative language expressing feelings such as hopelessness or guilt.
Loss of interest - Hobbies and activities they once enjoyed may no longer appeal. Their libido may also decline.
Low energy and fatigue - They may struggle to get up in the morning and feel sleepy or drained throughout the day.
Increased appetite and cravings - Comfort foods, especially carbohydrates and sweets, may become more tempting. This can lead to winter weight gain.

Key difference:
People with SAD often sleep more and eat more, while those with non-seasonal depression may do the opposite.

It’s important to remember that the severity of SAD varies from person to person.

How is SAD diagnosed?

There’s no single test for SAD. A GP will usually ask about your mood, sleep, appetite and energy levels, and will typically want to see the same pattern of symptoms recurring at the same time of year for two winters or more before making a diagnosis — it’s this recurring seasonal pattern that distinguishes SAD from other forms of depression.

SAD treatment

Treatment usually starts with light therapy, using a 10,000 lux lightbox for around 20 to 30 minutes each morning to help reset your circadian rhythm. Talking therapies such as CBT can help you manage low mood and unhelpful thought patterns, and in more persistent or severe cases, a GP may also prescribe antidepressants. Starting treatment before winter sets in, rather than waiting until symptoms become severe, tends to work best. See the NHS SAD page for more on what each option involves, or speak to a GP if you think you’d benefit from support.

How to support someone you know with SAD

There are many small, meaningful steps you can take to help someone you know who is experiencing SAD.

1. Encourage balanced, mood-boosting meals

Prepare or recommend meals rich in vitamins, lean proteins and complex carbohydrates that support serotonin, such as oatmeal, omelettes, lean chicken, nuts and popcorn.
Limit caffeine where possible — it can heighten anxiety and disrupt sleep.

2. Remind them to stay hydrated

Drinking enough water helps prevent fatigue and headaches, which can worsen low mood.

3. Help them get more daylight

Daylight helps reset the body clock and boosts mood. Try suggesting:

A lunchtime walk
Outdoor weekend plans
Sitting near windows during the day

If you're nearby, join them — even a 20-minute walk helps.

4. Suggest regular, gentle exercise

Movement increases endorphins and improves energy levels. This might include:

Walking or cycling to work
Home workouts
A weekend hike
Even housework

Any activity that raises the heart rate can make a difference.

5. Plan things for them to look forward to

Having enjoyable activities ahead can boost positivity. You might suggest:

A concert or comedy night
A day trip
A short break somewhere sunnier
A cosy meal out

Choose options that feel manageable and uplifting.

6. Talk openly and look for triggers

Ask how they’re feeling in a gentle, non-judgmental way. If they seem irritable, stay patient. Over time, you might spot patterns that help you both manage tough moments more effectively.

7. Encourage a healthy sleep routine

A structured bedtime routine is essential when sunlight is limited.

Help them by suggesting:

A calm, dark and tidy sleep environment
Reduced screen time before bed
Avoiding caffeine later in the day
A relaxing wind-down routine such as reading or a warm bath

8. Make their environment brighter

Help them maximise exposure to natural light:

Keep curtains and blinds open
Encourage sitting near windows
Keep rooms bright and uncluttered

Light therapy boxes (10,000 lux) can reduce symptoms for up to 80% of people, often within 3–4 days.

9. Explore talking therapies and professional support

Talking therapies, such as counselling or Cognitive Behavioural Therapy (CBT), can help someone understand their thoughts, manage difficult emotions and build healthier coping strategies. Encourage them to book an appointment with a Doctor Care Anywhere GP or Mental Health Practitioner, who can offer personalised support and guidance. If appropriate, they can also signpost the person to HelloSelf, where they can be matched with a leading therapist and use therapist-designed tools to help them remember, practise and apply what they learn between sessions.

10. Encourage them to speak to a GP or Mental Health Practitioner if symptoms worsen

If their symptoms become severe, persistent or begin to interfere with everyday life, encourage them to speak to a Doctor Care Anywhere GP or Mental Health Practitioner. They can assess symptoms, provide tailored advice and arrange further support if needed. In some cases, medication or structured therapy may be recommended — particularly if SAD returns each year. Starting treatment before winter can often be the most effective approach.

It’s completely natural to worry about someone you know if they’re not acting like themselves. Your support, understanding and patience can make a meaningful difference. With the right steps and early action, it’s possible to reduce the impact of SAD and help them feel brighter during the darker months.

Seasonal Affective Disorder is a type of depression that follows a seasonal pattern, usually worsening during winter when daylight hours decrease.

Common signs include low mood, tiredness, increased sleep, cravings for carbohydrates, irritability, lack of motivation and reduced interest in activities.

SAD is usually caused by reduced sunlight in autumn and winter, which can disrupt the body clock, lower serotonin levels and increase melatonin production.

You can help by encouraging daylight exposure, healthy routines, exercise, balanced meals, rest, talking therapies and by offering understanding and patience.

Yes. Light therapy using a 10,000-lux lamp is effective for many people and can improve symptoms within a few days of consistent use.

They should seek medical advice if their symptoms are severe, persistent, or return each winter. A GP can discuss treatment options such as therapy or medication.

No — SAD is a form of depression, not bipolar disorder. That said, some people with bipolar disorder do notice their mood shifts follow a seasonal pattern too. If you’re experiencing dramatic mood swings alongside seasonal low mood, it’s worth discussing this with a GP so they can work out what’s really going on.