According to a complex algorithm, which includes variables for weather conditions, post-holiday debt levels, time since Christmas, and a few other nebulous data points, the third Monday in January is the saddest day of the year.

And, according to the travel agency who commissioned the study, it is an ideal time to book a winter vacation.

It was a fun and clever little marketing schtick, and it has stuck in our popular conscience. The post-holiday doldrums can be a drag. But for people who suffer from depressive disorders, “Blue Monday” can feel like it lasts for weeks, and for others, years.

Depressive disorders can range from relatively minor episodes (though often still debilitating) to severe, protracted events. Different forms of depression are distinct from others, marked by unique symptoms and timelines, and may develop under specific circumstances.

Major depression — known as Major Depressive Disorder, or clinical depression. Symptoms include a persistent sad mood, loss of pleasure in hobbies and activities, decreased energy, and feeling hopeless. Symptoms must be present for at least two weeks for a diagnoses.

Psychotic depression — occurs during major depressive episodes when people experience a break with reality. Psychotic symptoms may include hallucinations, delusions, or paranoia.

Dysthymia — also known as Persistent Depressive Disorder, it is diagnosed after two years of low-level depressive symptoms. Dysthymia may also include episodes of major depression.

Postnatal depression — experienced by women after giving birth, it is caused by a complex mix of cultural factors, physiology, and experience. It is more severe and differs from “the baby blues”, a common outcome to the stress of adjusting to a new baby. Women can also suffer from perinatal depression, which covers the period of pregnancy through the first year after delivery.

Seasonal Affective Disorder — more common in the Northern Hemisphere where shorter days limit natural sunlight exposure seasonally, SAD is characterized by social withdrawal, lower energy, craving carbohydrates, and weight gain. The symptoms return predictably each year with the onset of winter.

There are other types as well, but all share, to a degree, the same symptoms, which manifest psychologically, physically, and socially. The Mayo Clinic lists the following symptoms for depression:

• Feelings of sadness, tearfulness, emptiness or hopelessness
• Angry outbursts, irritability or frustration, even over small matters
• Loss of interest or pleasure in most or all normal activities, such as hobbies or sports
• Sleep disturbances, including insomnia or sleeping too much
• Tiredness and lack of energy, so even small tasks take extra effort
• Changes in appetite — often reduced appetite and weight loss, but increased cravings for food and weight gain in some people
• Anxiety, agitation or restlessness
• Slowed thinking, speaking, or body movements
• Feelings of worthlessness or guilt, fixating on past failures or blaming yourself for things that aren’t your responsibility
• Trouble thinking, concentrating, making decisions and remembering things
• Frequent or recurrent thoughts of death, suicidal thoughts, suicide attempts or suicide
• Unexplained physical problems, such as back pain or headaches

If you are struggling with these symptoms, seek help. Depression responds well to treatment, which typically involves speaking with a mental health professional, and medication, if necessary. The earlier you seek help and support the better.