Seasonal Affective Disorder is a disease with many underlying causes. To understand what the problem really is we need to go back to the basics first, because what is this winter depression (or summer depression actually)? And how can people get depressed by the winter?

Seasonal Affective DisorderCredit:

Seasonal Affective Disorder (SAD)

Seasonal Affective Disorder, or recurrent winter depression is a sub-type of major depression. As stated in the DSM-IV (the Diagnostic and Statistical Manual of Mental Disorders) there: 'is a recurrent pattern of major depressive episodes during the winter and remission of the symptoms during the summer, in the absence of psychosocial stressors.'

In normal-human language this means that the depression is coming back with a pattern. For instance, every year you get the symptoms of seasonal affective disorder again. These symptoms, as depressive episodes, go away in the summer. It is important to state that there cannot be a psychosocial stressor. For instance, if you are a house-painter you may have less work in the winter which could put you on a lot of stress.

Seasonal Affective Disorder is a disease with many underlying causes. To understand what the problem really is we need to go back to the basics first, because what is this winter depression (or summer depression actually)? And how can people get depressed by the winter?

Seasonal Affective Disorder is a serious disease with good therapeutic options. Therefore it is important to acknowledge the symptoms and consult a doctor if you think something is wrong. Therefore, first of all, a list of symptoms.

Seasonal Affective Disorder Symptoms

The symptoms of SAD can be hard to note and that is the reason as well why there is a lot of misunderstanding about. Symptoms include:[704]

Frequent symptoms in SAD

  • Sadness. (96%) Sadness means a down mood, don't feeling like doing anything.
  • Decreased activity. (96%) The depressed mood causes to to be less active and a less undertaking. If you find yourself to rather hang in front of the tv than hanging out with friends, that is what I mean.
  • Social misfortune. (92%) Being socially less favorable, less comfortable.
  • Anxiety. (86%) This is when you feel anxious for a longer and more frequent time without a real reason.
  • Irritability. (86%) Being irritated easily (and fast) during the winter depression.
  • Occupational misfortune. (84%) Having no luck with your job, meaning you are less capable of succeeding in your job.
  • Daytime tiredness. (81%) Being tired at night is normal, but you shouldn't be tired during the day. This can be a symptom of SAD.

Fairly frequent symptoms in SAD

  • Increased sleep. (76%) Increasing sleep during the winter can be a sign of a winter depression.
  • Poor quality of sleep. (75%) If you notice you are sleeping worse during the winter, this can be a symptom of winter depression as well.
  • Increasing weight. (74%) This can be related to the less activity and the depressive mood. But in 74% of the people with SAD they have anm increased weight.
  • Carbohydrate craving. (70%) A fairly strange symptom, but people with SAD can have a bigger craving for carbohydrates. This is eating more grain products and eating more in general. Probably related to the increased weight as well.
  • Decreased libido. (68%) Always a symptom in depression. A decreased libido.
  • Increased appetite. (65%) As you can see, symptoms involving eating keep coming back.

Fairly infrequent symptoms in SAD

  • Suicidal thoughts. (35%) This is something to actually acknowledge. If there are thoughts like 'what am I worth?' and 'Would there be anyone missing me?' are called 'suicidal thoughts' and if the doctor isn't asking about them, you should actively tell them. Not only because they are one of the symptoms in a list, but also to adapt the therapy to it.
  • Decreased sleep. (31%) Another option rather than increased sleep can actually be decreased sleep.

Now the most frequent symptoms of SAD have been discussed let's dive in what the causes can be of SAD.

Seasonal Affective Disorder: The Causes of the Disease with a Seasonal Pattern

A disease with a seasonal pattern is always interesting. So there is something, other than psychosocial factors, that is influencing the onset of a depressive episode. For SAD there are several hypotheses according to the cause. As it is a psychiatric disease, the actual cause is always vague. That said, there has been done some impressive research done to show some possibilities of the cause of the disease.

SAD Causes: The Photoperiod and Melatonin

One of the common thoughts regarding the cause of Seasonal Affective Disorder is it has something to do with the shift of daylight (photoperiod). There have been done studies which show that there is a higher prevalence (number of patients with the disease) in northern countries.[705] This means that countries with a bigger difference in daylight between winter and summer have a higher prevalence of SAD. This of course could be a cue SAD has something to do with the daylight and the reason why the SAD Light Therapy boxes on Amazon should work.

After these studies there have been studies done as well which show there is an effect of photoperiod extension by giving light therapy. Later this have been shown wasn't effective if this is the only given therapy.[706] And even later another study showed there was only a little or no effect of the latitude in SAD.[707] All of this show that there is only a small correlation between photoperiod and SAD.

Melatonin is another example of a factor which seems important in SAD. On one hand, studies show that this isn't true. For instance, there isn't a difference in the 24-melatonin rhythm when you compare SAD patients with a healthy control group. Melatonin as a therapy is only effective in small (physiological) doses at set times to shift the circadian phase seem effective.[744]

On the other hand, the similarity of the melatonin rhythms can also be explained by the light patients always get because we have lights on during the day. If you measure the Dim Light Melatonin Onset (DLMO), the variances of melatonin with dim light only, there is a seasonal significant variation in patients with SAD, compared to control groups who don't have any variation at all.[745]

This means we are back to square one and the photoperiod could be a reason for seasonal affective disorder.

Seasonal Affective Disorder: sleepCredit:

SAD Causes: Circadian Phase Shift

To understand this hypothesis we first need to understand what the circadian rhythm is. The best example is to understand what happens when we put a human being in a completely time-free room. There are no windows, no newspapers and no other indications of what the time can be. If we do this, the human being will still live in a 24-hour shift similar to how we are living right now. (In fact the rhythm shifts from a 24-hour shift to a 25 hour when the patient is kept longer in this room. Just a side note).

This means our body somehow knows what time it is and how long a day is. This is called the circadian rhythm.

This circadian rhythm is controlled by bright light, which is the most important 'Zeitgeiber'. This light exposure can direct the circadian rhythm to shift in a certain direction.

In a study done by Lewy the hypothesis is that the circadian phase is delayed internally compared to the external clock or even the sleep-wake cycle. This is confirmed by other studies by a delayed DLMO. This would mean that the therapeutic effect of light therapy is correction the phase shift.

SAD Causes: Neurotransmitter Function

Talking about neurotransmitter and such can be really difficult to understand, so this would be a short note on the functioning of neurotransmitters in SAD patients. There are basically three important neurotransmitters in mood disorders: serotonin, dopamine and norepinephrine.


The neuroendocrine and behavioural responses to serotonin are abnormal in SAD patients, this could be a reason for the craving for carbohydrates, as serotonin has a function in feeding regulation.


There is not yet very consistent evidence for the role of dopamine in SAD, but there is some indirect evidence.


There are various lines of evidence that show there is a decreased arousal system. So where healthy people are awake, the SAD patients are still not aroused.

SAD Causes: Genetics?

There are not really conclusions you can draw from the research done right now. There are conclusions, but they are preliminary as they should be replicated with much bigger samples.


To conclude, there is a lot to say about possible causes for Seasonal Affective Disorder, but there is no definitive answer. All of them are hypotheses, as with a lot of psychiatric disorders.