I presume you're talking about something like Nytol or a similar over the counter sleeping pill: they're mildly effective if you aren't used to them; they generally make you feel quite relaxed and your body a little heavier. Doubling the dose won't be particularly efficacious. Although if it's a prescription sleeping pill like temazepam I certainly wouldn't recommend doubling (though given that you say they're 50mg tablets I doubt it is). If you take too many you'll just feel very heavy and groggy, a little like being drunk.