Async update web from controls on drop down list selected index change

Hey all, I need to asynchronously check a sql database every time someone changes a dropdownlist selected index, and then update a label with the count. Sounds simple. But It's not working for me. Here's what my web form looks like:
<asp:TableHeaderCell> <asp:UpdatePanel runat="server" ID="ddlPanel"> <ContentTemplate> Select Time: <asp:DropDownList runat="server" ID="restDDL" OnSelectedIndexChanged="restDDL_SelectedIndexChanged" AutoPostBack="true"> <asp:ListItem>5:00 pm</asp:ListItem> <asp:ListItem>5:30 pm</asp:ListItem> <asp:ListItem>6:00 pm</asp:ListItem> ...