Sicknesses like the common cold, flu, asthma, acid reflux (GERD), sinusitis, and even stomach issues often worsen at night due to circadian rhythms, hormone changes (lower cortisol), lying down causing mucus buildup (postnasal drip), and less distraction, intensifying symptoms like cough, congestion, and pain as your body's immune response peaks and gravity affects mucus drainage.
Whether you're dealing with the common cold, the flu or a stomach bug, you've probably noticed that your symptoms feel worse at night. You're not imagining things. Research suggests that your body's circadian rhythms—as well as some other factors—can exacerbate your symptoms after sundown.
The circadian rhythm will tell your body when to rise and shine, and when to prepare for sleep. When your body recognizes it's almost bedtime, it causes immune cells to become more active. These cells identify germs, causing inflammation to fight off infection, which in turn makes your symptoms feel more severe.
Think headache, high fever, aching body, chills and extreme tiredness. On day two, you'll feel even worse – and day three is worse still. For most people, it's day four when you'll start to feel better. Be careful, though.
More cortisol circulates in your blood during the day, which suppresses your immune system. This means that your white blood cells, which are responsible for fighting infections, are less active during the day. At night, there is less cortisol in your blood.
Doctors said that the surge in flu cases is largely related to a new mutation of the influenza A virus called "subclade K" — a strain that is not well covered by this year's flu vaccine.
Signs and Symptoms
A new influenza A(H3N2) virus subclade J.2.4.1, also called "H3N2 subclade K," was first identified by CDC through analysis of sequence data in August 2025. It emerged after the selection of viruses to be used in this season's influenza vaccines was made in February 2025.
The surge has been dominated by a strain of the Influenza A type virus known as Subclade K, which medical professionals are calling a “super flu.” Medical staff at the University of Kansas Health System recommend getting the flu vaccine if you haven't yet, even though it is not an exact match to this variant.
If you have a severe infection or are at high risk of complications from a flu infection, your healthcare professional may prescribe an antiviral medicine to treat the flu. These medicines can include oseltamivir (Tamiflu), baloxavir (Xofluza) and zanamivir (Relenza). You take oseltamivir and baloxavir by mouth.
The virus is still finding its new norm. Even so, a few things about this season's ongoing torment are clear. Much of the blame rests on the season's dominant flu variant—subclade K, which belongs to the H3N2 group of influenza.
Flu symptoms to know
H3N2 subclade K, the mutated version of the flu virus responsible for severe infections this winter, is estimated to last about seven days. However, for some, the so-called 'super flu' is more severe and can linger for weeks, subsequently taking longer to recover from.
The symptoms and severity of H3N2v illness have been similar to seasonal flu, including fever, cough, runny nose, and possibly other symptoms, such as body aches, vomiting, or diarrhea.
Flu A tends to be more aggressive and cause more severe symptoms compared to flu B. It's more common in adults. In contrast, flu B is more common in children. While it typically causes mild illness, type B can be more severe in children under age 5 and older adults.
The common cold, the flu, RSV, and COVID-19 are viral respiratory infections that spread easily during certain seasons.
The most common symptoms of RSV include:
You can't tell if you have COVID-19 or the flu just by symptoms because they overlap so much (cough, fever, fatigue, etc.), so testing is essential to know for sure, especially since early treatment for either virus can prevent severe illness. Get tested with a medical professional using a test that checks for both viruses to get a quick diagnosis and start treatment, as COVID-19 can have unique symptoms like loss of taste/smell and potentially lead to Long COVID.
While some people find flat, cool Coke or other sugary sodas temporarily helpful for nausea (especially for replacing sugars), many doctors advise against it, suggesting caffeine and carbonation can worsen symptoms, with flat ginger ale or water often better choices for settling an upset stomach. It's a mixed remedy, and if you're vomiting, hydration with clear fluids is key, but listen to your body as it might crave simple sugars.
Nausea at night is often a symptom of an underlying condition. Some of the most common causes include acid reflux, anxiety, medication side effects, peptic ulcers, or pregnancy. Nighttime nausea is often treatable with self-care or a visit to the doctor.
Avoid salty foods, alcohol, coffee and sugary drinks, which can be dehydrating. Ice chips are another simple way to stay hydrated and calm a scratchy throat. Gargle with salt water.
Sleeping More When You're Sick Is *Totally* Normal—And Evidence of Your Immune System in Action. The reason why being sick makes you tired (and why you sleep so much when you're sick) is because your immune system needs sleep to recover.
Day 4-5: Symptoms become more pronounced; fever, cough, and fatigue intensify. Day 6: Potential appearance of gastrointestinal symptoms like nausea or diarrhea. Day 7: Heightened symptoms which may include difficulty breathing and persistent chest pain.
The fluid flushes out
With a very densely packed, wet and high-pressure environment, your nose flows like a tap. Your runny nose is an easy way for your body to clear itself from all the used white cells, virus and excess fluid. You'll notice that your snot turns more green and less clear the longer the cold goes on.
Common signs and symptoms of the flu include: