The first Monday back after the holidays can be exhausting. Your brain is still in holiday mode and can’t quite remember who you are, what you do, or why your inbox exists. And for many people, that “Monday morning” feeling doesn’t disappear after a day or two; it can linger.
Post-holiday fatigue isn’t just in your head; there’s real science behind it. What is essential to understand is the difference between normal “reset” tiredness and fatigue that could point to an underlying issue, such as low iron or vitamin D levels.
If you’re concerned, Smith Biomed offers at-home Iron (Ferritin) deficiency tests and Vitamin D deficiency tests, making it easy to check your levels and take informed next steps.
The Post-Holiday Crash
During the holidays, your brain is swimming in stimulation. Social events, late nights, food, alcohol, travel, and a break from routine all increase dopamine and serotonin, the brain’s “feel-good” chemicals.
When the holidays come to an end, that stimulation drops abruptly. Neuroscientists often refer to this as a contrast effect: the bigger the high, the flatter the low feels afterwards. Your nervous system now needs some time to recalibrate.
For some people, that recalibration takes days. For others, it drags on for weeks.
Three Reasons January Fatigue Hits Hard
1. Nervous System Overload
Weeks of socialising and disrupted routines keep cortisol (your stress hormone) elevated. Once the pace slows, cortisol drops, and the result can feel like exhaustion, brain fog, and low motivation.
2. The Metabolic Cost of the Holidays
High sugar intake, alcohol, irregular meals, and late nights all affect blood sugar regulation and gut health. Even after you return to “normal eating,” your energy systems may still be playing catch-up.
3. Hidden Nutrient Depletion
Holidays often mask fatigue rather than fix it. Reduced workloads, flexible schedules, and adrenaline from novelty can temporarily mask low energy, but they don’t replenish depleted nutrient stores.
When normal demands return, deficiencies like low iron or vitamin D become harder to compensate for. Fatigue, brain fog, low motivation, and breathlessness may show up not because January is demanding but because your reserves are already low.
The Low Energy Checklist
Not all tiredness is a red flag. Here’s an easy way to gauge what’s normal and when tiredness needs further action.
The “Normal” Zone (1–14 days)
- Sluggishness
- Mild irritability
- Trouble concentrating
- Missing holiday freedom
This is typically a post-holiday adjustment. Rest, routine, and time usually resolve it.
The “Watch” Zone (2–4 weeks)
- Ongoing fatigue
- Low motivation
- Reduced enjoyment in things you usually like
This may suggest nutrient depletion.
The “Action” Zone (4+ weeks or severe symptoms)
- Persistent low mood, numbness, or hopelessness
- Shortness of breath, chest pain, or unexplained weight changes
- Brain fog affecting work or daily tasks
- Social withdrawal
At this stage, it’s worth checking in with your GP or healthcare professional and considering testing for key nutrients such as iron and vitamin D.
Could Low Iron or Vitamin D Be Contributing?
Iron and vitamin D deficiencies are common and often overlooked.
- Low iron can cause fatigue, breathlessness, poor concentration, and weakness, even before anaemia develops.
- Low vitamin D is linked to low energy, muscle pain, frequent illness, and low mood.
Smith Biomed’s at-home Iron (Ferritin) deficiency test and Vitamin D deficiency test allow you to check your levels easily, without needing an immediate GP appointment. Results can help guide whether supplementation or medical follow-up is needed.
Always discuss abnormal results with a healthcare professional.
Test, Don’t Guess
If January tiredness feels more than “just getting back into routine,” it’s worth listening to your body. Testing iron and vitamin D levels can provide answers, and this is the first step toward feeling better.
You shouldn’t be feeling tired all the time. Sometimes this post January slop is the push you need to check your health.