This is one of the most common broadband patterns we see, and it's almost always one of two things. Both produce the same symptom — perfect during the day, rubbish from about 6pm onwards — but they're entirely different problems with different fixes.
Cause 1: Contention (your ISP's network is busy)
Every broadband line shares some bit of infrastructure with other customers — the fibre back from the cabinet, the data link from the exchange to the ISP's core network, the ISP's connection to the rest of the internet. When loads of people in your area start streaming Netflix at 7pm, that shared capacity gets used up.
Most providers will throttle individual customers under congestion rather than let the network fall over. So your line, technically capable of 75Mbps, might be throttled to 20Mbps between 6pm and 11pm because the ISP's network in your area is saturated.
This is most common with:
- Cheap "no-frills" ISPs (TalkTalk, Plusnet at peak times, some Vodafone deals)
- Older FTTC lines in high-density areas (mansion blocks in central London where the cabinet serves a lot of flats)
- Areas where Openreach hasn't yet built out enough cabinet capacity for the demand
How to spot ISP congestion
Log into your router's admin page (usually 192.168.1.1) and look at the broadband statistics. During a slow evening:
- Sync speed: probably the same as normal (e.g. 76Mbps).
- SNR margin: high and steady (12dB+).
- Throughput on speed test: way below sync speed (e.g. 20Mbps).
If sync is healthy but actual speed isn't, your line is fine — the throttling is happening further up the chain. That's ISP contention. The fix is to switch ISP. Or, if you're on FTTC and FTTP is available, upgrade to fibre (FTTP has much higher contention headroom).
Cause 2: REIN (something in your building is pumping electrical noise into the phone line)
REIN stands for Repetitive Electrical Impulse Noise. Cheap electronics — particularly LED dimmer switches, low-quality phone chargers, LED filament bulbs, fish tank heaters, electric blinds — generate electrical noise on their mains lead. That noise propagates back through the building's wiring and, if your phone cable runs near the mains, gets coupled into the phone pair.
The phone line carries broadband on frequencies that overlap with the noise. Result: your modem sees the line as much noisier than it really is, and either drops sync entirely (broadband cuts out) or retrains to a lower speed (broadband gets slow).
The "evening at 6pm" pattern is the giveaway. People come home from work. Hallway lights go on. Phone chargers plug in. Cheap LED bulbs in the bathroom switch on. All the kit that generates REIN starts running at the same time every day. Your line tanks.
How to spot REIN
Same router stats page. During a slow evening:
- Sync speed: dropped (e.g. retrained from 76Mbps down to 24Mbps).
- SNR margin: jumping around — 12dB one second, 3dB the next, back to 12dB. Sometimes the router page only shows a single number — look at it over a minute, it'll be visibly unstable.
- Throughput: matches the lowered sync.
If sync has dropped and SNR is unstable, you've got REIN. The line is technically capable of 76Mbps; something in the building is preventing it.
Sarah in Stoke Newington had this exact problem. Brilliant connection until about 7pm every evening, then it tanked. Sky said the line was fine. She'd done dozens of speed tests — most showed 60+Mbps, but the evening ones showed 8.
I turned up at 8pm deliberately. Plugged the test set into the master socket. Watched the SNR margin. It was sitting at 14dB, then dropping to 3dB, then back to 14dB, in a regular pattern every 7 seconds. Classic REIN signature — something electrical was switching on and off, the 7-second pattern matching a thermostat cycle on something.
It took us about 20 minutes to find it. Her downstairs neighbour had installed a cheap LED dimmer switch in their hallway — generic online unit, no proper EMC filtering, dumping electrical noise straight into the building's shared phone wiring. The 7-second cycle was the dimmer thermal-cycling on a slightly-too-low-rated triac.
The fix was a small ferrite choke clipped onto the dimmer's mains lead inside the wall back-box, fitted by me with the neighbour's permission. Total noise gone. Her evening speed went back to 60Mbps. She'd been on hold with Sky six or seven times over six weeks; the actual fix took an hour and most of it was diplomacy with the neighbour.
How to find a REIN source yourself
This is genuinely DIY if you're patient. The general approach:
- Wait until the broadband is in its bad state (usually evening).
- Unplug every non-essential device in your house — every charger, every LED lamp, every TV on standby, the lot. Leave only the router on.
- Wait 5–10 minutes for the modem to potentially retrain.
- If broadband recovers, you know the cause is in your flat. Plug things back in one at a time, waiting a few minutes between each, until the noise returns. The last thing you plugged back in is the culprit.
- If broadband doesn't recover after everything in your flat is unplugged, the source is in a neighbour's flat or in the communal wiring. You need either neighbour cooperation or an engineer with a noise meter to find it externally.
Common culprits, in order of frequency I see them:
- Cheap LED dimmer switches (especially generic ones from online marketplaces — branded MK or Crabtree dimmers are usually fine)
- Cheap USB chargers left plugged in (the generic multi-pack online ones are notorious)
- Filament-style LED bulbs with cheap drivers
- Fish-tank heaters with bimetallic thermostats
- Electric blinds and remote-control curtain motors
- Christmas lights (yes, even non-Christmas-themed ones if they're cheap LED strings)
The fixes
For ISP contention: switch provider or upgrade to FTTP if available. Sticking with the same ISP rarely solves congestion problems — it's a network-capacity issue, not a service issue. Specifically, BT (FTTC), Sky (FTTC) and TalkTalk are the most contention-prone in central London; A&A, Zen and BT (FTTP) tend to be better in my experience.
For REIN inside your own flat: identify the device and either replace it with a better-quality branded one or fit a ferrite choke on its mains lead.
For REIN from a neighbour's flat: this is the awkward one. The polite approach: explain the problem, offer to swap in a replacement bulb/dimmer/charger that doesn't cause it. Most people will cooperate if it's framed as "this small change and we both stop having broadband issues." If they won't, you can fit a filtered faceplate on your end of the line (NTE5c with an integrated VDSL filter) — it won't fully solve it but it'll mitigate.
For REIN from the building's shared wiring: engineer job. We trace the source with a directional noise meter, identify which flat or which bit of shared wiring is the source, and fit the fix there.
What ISPs will tell you (and why it's not enough)
"Your line tests fine." This is the line you'll get from Sky, BT, TalkTalk and the rest when they run a remote test. It's not lying — but their automatic tests run at low frequencies and during quiet periods. They don't see REIN unless they happen to test at exactly the moment the noise is happening.
If you tell the ISP "it's bad every evening between 6pm and 11pm" and they keep telling you the line tests fine, they're testing at the wrong time. You can ask them to schedule a test during the bad window — some ISPs will, some won't. Otherwise, get an engineer with a real-time SNR meter to come round at the right hour.
Common questions
Why does my broadband drop at the same time every evening?
Two causes — ISP network congestion (everyone's streaming at peak time and your ISP throttles) or REIN (something electrical in the building is pumping noise into the phone line). The router stats tell you which: stable sync but slow speed = ISP. Dropped sync with unstable SNR = REIN.
Can I tell which it is myself?
Yes, partly. Log into your router during a drop. If sync speed is unchanged but throughput is low — ISP contention. If sync has retrained lower and SNR margin is jumping around — REIN.
How do I find the device causing REIN?
Unplug everything non-essential during a bad period and see if broadband recovers. If it does, plug things back one at a time until the noise returns. The last device plugged back is your culprit. If unplugging everything in your flat doesn't help, the source is a neighbour's flat or the communal wiring — that's engineer territory.
Broadband good by day, terrible by night?
Take a screenshot of your router's broadband stats page during a bad moment, ring me, and I'll tell you on the phone whether it's worth getting an engineer out (REIN) or switching ISPs (contention).