Hi SallyKat, my Mum sleeps for England too and has done for about 4 years now, before we even realised dementia was in the mix. Early days I thought it was the tablets she was on but it really wasn't.
She wakes up mid to late morning, sleeps after her change, wash and breakfast and then she's asleep until about 5-6 pm when we have the next cycle of cups of tea,change, dinner.
After that she'll snooze and wake a little until "proper bedtime" after which she'll sleep through until the next day.
We did have a couple of years of sleepless nights and waking to try to wander round her room and get herself on to the commode - all with accompanying falls and trips to A&E, Ambulance call-outs, but she's been bed-bound for 18 months now (she's also physically disabled with a dreadful back condition and probably dementia progression), so in a way I am relieved that she can no longer fall and hurt herself but so very sad she can't move about of her own free will.
Her sleeping such a lot does mean it's quite stressful trying to get enough food and liquid into her each day, bed-bathing is dependent on when she's awake too as it's really not worth trying to wake her up, she really is exhausted.
Unfortunately.....this also means it's a problem trying to get the chores done - nothing noisy like hoovering or clattering about doing the dishes but hey-ho, there have to be some advantages!
I would agree with having the dizziness checked out though, Mum has had several batteries of tests done over the years and has nothing physically wrong.
Her GP should be able to arrange for blood tests to be done at home if it's a struggle to get her to the surgery.