[saghm]

When this happens to me, it always feels like I'm about to fall forward. The best I can describe it is as the feeling that I just took a step forward but instead of my foot finding the ground, the surface I was walking on ended (as if I stepped off a ledge or something).

[rzzzt]

For me it's the opposite: it feels like someone pushing me out of a tree swing or a chair that has no backrest (I'm sleeping on my side).