Your mood can genuinely be affected by shorter days and less daylight. The NHS describes this pattern, low mood that comes on in winter and lifts again by summer, as seasonal affective disorder, or SAD (Source: NHS, 2026). It's thought to be linked to reduced daylight affecting brain chemicals involved in mood and sleep, including melatonin and serotonin. For some people this is a mild, manageable dip. For others it's more disruptive, and it's worth knowing the difference.
Who this is for
You've noticed a pattern, maybe you feel low, tired, or unmotivated every autumn and winter, and more like yourself again by spring, and you want to understand whether that's normal or worth doing something about. This is also for anyone who just wants to know what seasonal low mood actually is before assuming it's something else.
What seasonal low mood actually looks like
The NHS describes SAD as low mood, along with a loss of interest in things you'd usually enjoy, that follows a seasonal pattern, appearing in winter and easing in summer (Source: NHS, 2026). Alongside low mood, common symptoms include feeling more tired than usual, sleeping more, craving carbohydrates and gaining weight, finding it harder to concentrate, and feeling irritable or low in confidence.
A GP will usually ask about how your mood has changed over time, and may diagnose SAD once this pattern has repeated for two years or more (Source: NHS, 2026). You don't need to wait that long to get support, though. Mind is clear that you don't have to wait to see if a pattern develops before seeking help (Source: Mind, 2026).
What tends to help
The NHS and Mind both point to talking therapies, particularly cognitive behavioural therapy (CBT), as a recommended treatment, alongside antidepressants where a GP judges them appropriate (Source: NHS, 2026). Some people start medication a few weeks before their symptoms usually begin, on their GP's advice (Source: Mind, 2026).
Light therapy using a "SAD lamp" or light box, designed to simulate daylight, is something many people find helpful, though the NHS is upfront that the evidence for it isn't strong enough for it to be routinely available on prescription (Source: NHS, 2026). Mind adds that light boxes are bought privately rather than provided by the NHS for this reason, and that anyone on medication that increases sensitivity to light, or with certain eye conditions, should check with a doctor before trying one (Source: Mind, 2026).
When it's worth seeing a GP
If low mood is following a seasonal pattern and affecting your day to day life, whether that's your sleep, your concentration, or how you're managing at work, it's worth speaking to a GP rather than waiting to see if it passes. This is especially true if your current approach, whatever that's been, doesn't seem to be helping. Mind notes that if one treatment doesn't work, a GP should be able to suggest an alternative, and can refer you on for further support if needed (Source: Mind, 2026).
It's also worth remembering that low mood in winter can overlap with ordinary stress or a difficult period, rather than being seasonal in the way SAD is. If you're not sure which applies to you, that's a reasonable thing to raise with a GP too.
Seasonal affective disorder is a pattern of low mood that appears in winter and lifts in summer, and the NHS links it to reduced daylight affecting mood-related brain chemicals such as melatonin and serotonin (Source: NHS, 2026). Common symptoms include low mood, low energy, oversleeping, carbohydrate cravings, and difficulty concentrating. Recommended treatments include talking therapies such as CBT and, where appropriate, antidepressants, while light therapy helps some people even though the NHS says the evidence for it isn't yet strong enough for routine prescription (Source: NHS, 2026; Mind, 2026). You don't need to wait for a clear seasonal pattern to build before speaking to a GP if low mood is affecting your day to day life (Source: Mind, 2026).
Questions people often ask
A mild dip in mood or energy as days shorten is common. It becomes seasonal affective disorder when low mood follows a clear yearly pattern, winter low, summer better, and is diagnosed by a GP based on that pattern over time (Source: NHS, 2026).
It's a form of low mood that comes on in winter and eases in summer, thought to be linked to reduced daylight affecting brain chemicals involved in mood and sleep (Source: NHS, 2026).
The main difference is the seasonal pattern. Ordinary low mood can happen at any time and is often tied to a specific situation, while SAD follows the same yearly rhythm, typically starting in autumn and lifting by spring (Source: NHS, 2026).
If low mood is affecting your day to day life, whether that's sleep, concentration, or how you're coping at work, it's worth speaking to a GP. You don't need to wait for a clear seasonal pattern to develop first (Source: Mind, 2026).
Many people find light boxes helpful, but the NHS says the evidence isn't strong enough for it to be routinely available on prescription, which is why light boxes are usually bought privately (Source: NHS, 2026; Mind, 2026).
This article is for general information only and is not a substitute for professional medical advice. If you are concerned about a specific symptom, please see a GP or other qualified healthcare provider.