The sun is shining, the days are growing longer and the open-air season has begun.
It is spring, but not everyone feels energetic. For some, it is a season of leaden limbs and fits of yawning.
"Springtime lethargy is a vegetative reaction to the changes in nature," said Angela Schuh, a professor of medical climatology at the University of Munich.
People, like animals, regulate their metabolism and hormone levels in tune with external stimuli such as light and temperature.
When it is cold and dark outside, the body protects itself. "Our core temperature in winter is a few tenths of a degree centigrade lower than in summer. This slows down metabolism so the body goes into a kind of mini-hibernation, Schuh said.
"During these months, the body produces more of the sleep hormone melatonin, resulting in a pronounced need to sleep.When spring comes, bringing longer and stronger hours of sunlight, the body must adjust.
Body temperature rises, blood vessels dilate, blood pressure sinks. The light causes the body to release more of the "activity hormone" serotonin.
"The body can't manage the adaptation processes overnight, though. It takes about two or three weeks," said Heidrun Holstein, a physician for the consumer advice centre of the German state of Baden-Wuerttemberg.
Factors other than processes in the vegetative (autonomic) nervous system can contribute to springtime lethargy.
Infections are common during changes of season. Large day-night temperature swings put added strain on blood vessels and circulation.
Depending on climatic conditions, springtime lethargy and the switch to daylight saving time may coincide.
Personal habits play a role too, such as how one organises one's day. Springtime lethargy symptoms can vary widely: "Common complaints are tiredness, dizziness, irritability, headaches, mild sensitivity to changes in the weather and a tendency towards a sad mood," said Michael Stimpel, an internist and a professor in the University of Cologne's medical department.
Particularly at risk, he said, are the elderly and frail, women, and people with unstable blood circulation or those who have had very little exercise over the winter.
Those who suffer from springtime lethargy can alleviate their symptoms using simple means.
"You can act preventatively knowing that light stimulates the production of the activity hormone serotonin," Stimpel said. "You can spend a lot of time outdoors or undergo light therapy with special lamps which use filtered light."
Sauna baths, Kneipp hydrotherapy and contrast showers stabilise circulation and steel the vascular system so it is less susceptible to temperature fluctuations. Exercise activates the whole body.
"Eating plenty of fresh fruits and vegetables supplies the body with an extra portion of vitamins and minerals," Holstein said.Drinking a sufficient amount of fluids was also part of a prevention program, she said.
This makes the body fit for spring from the inside out. When springtime lethargy is especially acute, it helps to take a short break and get some fresh air.
"Applying cold water on the forearms or a damp cloth to the forehead banishes the symptoms," Stimpel said.
Schuh recommends giving the body time to adjust. "Take it easy, and don't take the symptoms too seriously," she said.According to Holstein, "it's like mild jet lag - the body has been brought out of its accustomed rhythm."