Your boiler is currently operating in a programmed energy-saving mode, which is perfectly normal behavior rather than a system fault.

DIY fix possible

Check your external heating programmer or thermostat settings to see if a 'night setback' or 'eco' schedule is active.Adjust your timer or thermostat to your preferred daytime temperature settings if you wish to override the current mode.Refer to your boiler's user manual for instructions on disabling night-time economy settings if you prefer the heating to remain at a constant temperature.