Why can antihistamines make you sleepy? And tips for sleeping with hayfever and other allergies...
This is a summary version of a longer article "Allergies at night and how they affect sleep" written for Bensons for Beds.
What causes allergies?
Allergies occur when the body’s immune system reacts to a harmless substance as if it were a dangerous invader. The allergen triggers the release of specific antibodies – messengers which signal cells in the skin, nose, lungs, mouth gut and blood to produce histamines.
Histamine encourages the immune system to go to work – increasing blood flow, causing swelling, mucus production, itching, sneezing and watery eyes. These reactions are the body’s attempts to help expel the allergen.
How do allergies affect sleep?
Sleep can be impacted by allergies in both obvious and hidden ways. Firstly, symptoms such as itchy eyes, a runny nose, wheezing or sneezing can actively interfere with the process of falling asleep. If you’re in pain, or getting frustrated by the discomfort, it’s naturally very difficult to relax and fall asleep.
However, even when you’re fast asleep, allergies could still be disrupting sleep quality.
When we have an allergic reaction, our nasal passages become inflamed and congested. This congestion narrows the airways, which can cause wheezing and snoring. Breathlessness has been linked to higher levels of arousal during sleep – in other words, you’re more likely to have broken sleep, or to wake up feeling fatigued (McKeown et al 2021).
Snoring is usually harmless, but if it leads to temporary pauses in breathing, or ‘apnoeas’ this can disrupt the deeper stages of sleep. Someone suffering from obstructive sleep apnoea is frequently pushed into lighter, less restorative sleep and so they will wake up feeling very tired after normal amounts of sleep. (You can read more about sleep apnoea here.) Although allergies are not thought to cause sleep apnoea, they can worsen its severity.
Congestion in the nasal passages can also increase mouth breathing, rather than nasal breathing. Signs of mouth breathing during sleep include a dry mouth, headaches, daytime sleepiness, bad breath and tooth decay (Udaka et al 2007). Nasal breathing helps to filter the air we breathe, and so mouth breathing can actually make us more vulnerable to breathing in allergens which can trigger a worsening of symptoms.
How do anti-histamines affect sleep?
Antihistamines work by blocking histamines attachment to receptors, which prevent the compounds from carrying out their functions. This means they can relieve some of the sleep-disrupting symptoms of allergies, such as itching, sneezing and wheezing, which can make it easier to relax and fall asleep.
They can also cross the blood-brain barrier and inhibit one of the other functions of histamines, which is the role they play in regulating sleep and wakefulness. This disruption of the action of histamines in the brain results in drowsiness. This is a well known side effect of some older types of anti-histamine, such as diphenhydramine (Benadryl). This can be dangerous for anyone who needs to drive or operate heavy machinery.
Newer anti-histamines such as loratidine (Claritin) have different chemical structures which don't allow them to cross the blood-brain barrier as readily. For most people, one-a-day anti-histamines don't cause sleepiness - but it can still can be a rare side effect.
Some people build up a tolerance to anti-histamines when taken regularly; in other words, the medication may become less effective over time. Antihistamines can also block the action of an important neurotransmitter for learning and memory (acetylcholine), and some studies have suggested that long term use of anti-histamines could increase the risks of cognitive decline (e.g. Shelley et al 2015).
Why are allergies worse at night?
When you lie down to sleep, gravity can encourage congestion to settle in your nasal passages and throat, which can exacerbate coughing and wheezing.
Some people with allergies to dust mites may also get more exposure to allergens from their bedding or mattress as they get into bed. Certain types of pollen are also more likely to be released in the evening and early morning.
However, even if you stay standing, allergic reactions tend to follow a daily rhythm which sees symptoms worsen in the early morning and late at night (Nakao et al 2015).
All of our body’s cells have instructions to operate on a 24 hour rhythm of action and recovery. These circadian rhythms are designed to help us be more active during the day, and to sleep and recover at night.
The skin epithelial cells which form our barrier to the outside world become more permeable overnight i.e. they are less protective, since traditionally we don’t get as much exposure to toxins in the evening, so it’s as if they have a bit of a rest. This makes us more susceptible to allergens through the skin and mucosal surfaces of the nasal passages. At the same time, certain immune cells which help to enact an allergy response actually become more active.
We are therefore more vulnerable to allergic reactions which enter the body via the skin and mucosal surfaces late at night, such as pollens and dust mites. Allergies can stimulate the release of histamine, which promotes wakefulness, and makes it harder to fall asleep and stay asleep.
If you are taking allergy medication, check with your doctor about the best time to take it. Some allergy medication is more effective in the evening than the morning (Storms et al 2004).
Interestingly, sleep loss and disruption to our typical circadian rhythms, such as through shift work or flying across time zones, may make us more susceptible to allergies (Nakao et al 2020).
Another reason that allergies may feel worse at night is because of the lack of distractions. When we’re busy during the day, it can take our mind off the symptoms, whereas at night, it’s easy to focus on the discomfort, and worry that an allergy will interfere with sleep. Hayfever has been linked to increased rates of anxiety and depression (Rodrigues et al 2022).
Tips for sleeping with allergies
1. Reduce exposure to allergens
This sounds obvious, but look for ways to avoid letting allergens into the bedroom.
Many people sleep with their pets in the bedroom without realising that they allergic to them. Pets may be lovable, but they tend to be disruptive to sleep, so I’d recommend making the bedroom a pet-free zone.
Dust and vacuum your bedroom regularly, and wash the bedding at least once a week in hot water to reduce the build up of dust mites.
If you suffer from hayfever, leave the windows closed if you can, and instead use a fan to keep air circulating.
Leave your shoes and clothes outside the bedroom and have a shower before getting into bed in order to wash off allergens such as pollen.
If you are allergic to pollen, check the pollen count each day during hayfever season, and consider limiting your time outside when the count is high.
Consider using hypoallergenic mattress and pillow protectors which are impermeable to dust mites.
You could also buy an air filter which removes pollen from the air.
2. Sleeping position
To limit nasal congestion, you may find it helpful to sleep on your side, or to lie on your back with your head propped up by pillows.
3. Allergy medication and treatment
Anti-allergy medication is sold over-the-counter at most pharmacies. Check with your pharmacist or doctor if you are uncertain whether this would be suitable for you – it will depend on the nature of your allergy, and any other medication you are taking.
Most allergy medication contains anti-histamines, to block the effects of histamine. Side effects may include a dry mouth, nose or throat, headaches, daytime fatigue, visual disturbances or confusion. Some anti-allergy products also include decongestants, which can have a stimulant effect on the heart. Always check the instructions on the packet to see how long it is recommended to take the medication for, and how often.
If you are struggling with allergy symptoms, speak to your doctor. They may refer you to a specialist who can discuss alternative treatments such as allergy shots, or immunotherapy where you gradually manage exposure to the allergen over time.
4. Develop consistent sleep habits, and protect time for sleep
There is some evidence that short sleep, or disrupted sleep, can worsen the severity of allergies (Mann et al 2020). Of course, this could lead to a negative cycle, where lack of sleep makes symptoms worse, which then makes it even harder to catch up on sleep.
While you may not be able to control all your allergy symptoms, you can take other steps to improve your sleep quality such as waking up at the same time every day, finishing your evening meal at least 2 hours before getting into bed, having a consistent bedtime routine and avoiding caffeine in the afternoon.