For years scientists have been striving to better understand the link between tiredness and headaches, or rather a person’s body clock—also called the circadian rhythm—and their symptoms.

Now a series of studies have revealed two types of headaches that are closely linked to a patient’s body clock and, more specifically, the time of year and the time of day a person is more likely to come down with one.

It’s a breakthrough which could potentially save the economy millions—if pharmaceutical brands can use the information to produce more efficient drugs.

Few studies have been completed to establish the cost to businesses of employees suffering with migraines, but one analysis completed in 1999 suggested that the U.S. economy was hemorrhaging $13 billion every year, while peers in Europe were losing €27 billion annually. The Harvard Business Review estimates that these costs are likely to have only gone up since the analysis was completed.

A study led by Dr Mark J. Burish, associate professor in the Vivian L. Smith Department of Neurosurgery with McGovern Medical School at UTHealth Houston, has now analyzed 72 previous headache studies  to see if timing could be pinpointed as to when headaches hit.

The two types of headaches they focused on are cluster headaches and migraines. Cluster headaches are the lesser-known of the two, and cause sufferers intense pain in or around one eye on one side of the head. The rare illness has already been linked to the body clock, as the headaches have been reported to frequently occur in the middle of the night, though more information about seasonality was previously unknown.

When am I most likely to get a cluster headache?

Dr Burish’s results—compiled with the help of 10 other scientists—were published by Neurology, and established that cluster headaches peaked not just at night, but specifically between the hours of 9 p.m. and 3 a.m. Not only did the group uncover peak hours, it also discovered that cluster headaches have circannual peaks that get worse in the spring and autumn.

Dr. Burish’s analysis also revealed that cluster headaches are not only dependent on the time of day but are written into a subject’s very genes.

At a cellular level, five of the nine cluster headache susceptibility genes are clock-controlled genes (CCGs) that are related to the regulation of the body’s rhythm.

When am I most likely to get a migraine?

Less was previously known about the link between migraines and the body clock, Dr. Burish added. However, results analysis of previous studies found that people struggled with migraines from daylight hours up until around 11 p.m.

“Cluster headache is known to be circadian, but it was surprising how circadian migraine is,” Dr. Burish told NBC News.

There was some respite for patients between the hours of 11 p.m. and 7 a.m., though their symptoms were reportedly worse in the months from April to October. As with cluster headaches, susceptibility to migraines is written into a person on a cellular level, with the study finding that 110 of the 168 migraine-inducing genes were CCGs.

The study also confirmed some key differences between migraines and cluster headaches. If migraines are left untreated they can last for several days, but they tend to happen only once in that time period. Conversely, cluster headaches can disappear and come back up to eight times a day, happening in 15- to 180-minute bursts.

Likewise, cluster headaches tend to come back in the same place around the temple or eye, with patients reporting they felt restless and tried to relieve the pain by walking around a room. Migraines can differ in where they are located, but tend to force someone suffering with them to rest in a darkened, quiet room.

“Medications that target the circadian cycle might be a new type of treatment we can offer these patients,” Dr Burish said. “We weren’t sure if looking at circadian targets of therapy for migraine would actually do anything, but after putting all of this together, we are more confident that it could be.”

Another scientist on the project, Dr Seung-Hee Yoo, associate professor in the department of biochemistry and molecular biology with the UTHealth Houston medical school, added: “A better understanding of headache timing might eventually lead to medications that prevent the headaches at those times or manage painful episodes better.”