The "acceptable range" (according to BT info) of speed for my Infinity broadband is 20 to 38 Mbps. Each morning I have to reset/restart my BT Hub 3 as the indicated download speed (on the BT Wholesale speed test) is a mere 1 to 3 Mbps. Why should this be?
In order to rule out the Homehub as potentially being the device responsible for your speed loss, you can connect the Openreach modem directly to the PC and see if the speed still falls overnight.
To do that follow these steps:
Step 1: Remove the homehub from the modem cable
Step2: Connect your pc/laptop to the modem via the cable instead
Step3: Open network and sharing centre on windows and click set up a new connection or network
Step4: Connect to the internet then click make a new connection Broadband PPPoE
Step5: Username email@example.com and the password as BT
Step6: connect, If it doesn't,keep pressing try again until it does.
However, please note that this will stop any other access apart from the PC